    {-# LANGUAGE ConstraintKinds
               , EmptyDataDecls
               , FlexibleContexts
               , FlexibleInstances
               , FunctionalDependencies
               , GADTs
               , MultiParamTypeClasses
               , OverloadedStrings
               , UndecidableInstances
               , ScopedTypeVariables
               , InstanceSigs
     #-}
    -- | This is an internal module, anything exported by this module
    -- may change without a major version bump.  Please use only
    -- "Database.Esqueleto" if possible.
    module Database.Esqueleto.Internal.Sql
      ( -- * The pretty face
        SqlQuery
      , SqlExpr
      , SqlEntity
      , select
      , selectSource
      , selectDistinct
      , selectDistinctSource
      , delete
      , deleteCount
      , update
      , updateCount
      , insertSelectDistinct
      , insertSelect
        -- * The guts
      , unsafeSqlCase
      , unsafeSqlBinOp
      , unsafeSqlBinOpComposite
      , unsafeSqlValue
      , unsafeSqlFunction
      , unsafeSqlExtractSubField
      , UnsafeSqlFunctionArgument
      , rawSelectSource
      , runSource
      , rawEsqueleto
      , toRawSql
      , Mode(..)
      , IdentState
      , initialIdentState
      , IdentInfo
      , SqlSelect(..)
      , veryUnsafeCoerceSqlExprValue
      , veryUnsafeCoerceSqlExprValueList
      ) where

    import Control.Applicative (Applicative(..), (<$>), (<$))
    import Control.Arrow ((***), first)
    import Control.Exception (throw, throwIO)
    import Control.Monad (ap, MonadPlus(..), liftM)
    import Control.Monad.IO.Class (MonadIO(..))
    import Control.Monad.Trans.Class (lift)
    import Control.Monad.Trans.Resource (MonadResource)
    import Data.Acquire (with, allocateAcquire, Acquire)
    import Data.Int (Int64)
    import Data.List (intersperse)
    import Data.Monoid (Last(..), Monoid(..), (<>))
    import Data.Proxy (Proxy(..))
    import Database.Esqueleto.Internal.PersistentImport
    import Database.Persist.Sql.Util (entityColumnNames, entityColumnCount, parseEntityValues, isIdField, hasCompositeKey)
    import qualified Control.Monad.Trans.Reader as R
    import qualified Control.Monad.Trans.State as S
    import qualified Control.Monad.Trans.Writer as W
    import qualified Data.Conduit as C
    import qualified Data.Conduit.List as CL
    import qualified Data.HashSet as HS
    import qualified Data.Text as T
    import qualified Data.Text.Lazy as TL
    import qualified Data.Text.Lazy.Builder as TLB

    import Database.Esqueleto.Internal.Language

    -- | SQL backend for @esqueleto@ using 'SqlPersistT'.
    newtype SqlQuery a =
      Q { unQ :: W.WriterT SideData (S.State IdentState) a }

    instance Functor SqlQuery where
      fmap f = Q . fmap f . unQ

    instance Monad SqlQuery where
      return  = Q . return
      m >>= f = Q (unQ m >>= unQ . f)

    instance Applicative SqlQuery where
      pure  = return
      (<*>) = ap

    -- | Constraint synonym for @persistent@ entities whose backend
    -- is 'SqlPersistT'.
    type SqlEntity ent = (PersistEntity ent, PersistEntityBackend ent ~ SqlBackend)

    ----------------------------------------------------------------------

    -- | Side data written by 'SqlQuery'.
    data SideData = SideData { sdDistinctClause :: !DistinctClause
                             , sdFromClause     :: ![FromClause]
                             , sdSetClause      :: ![SetClause]
                             , sdWhereClause    :: !WhereClause
                             , sdGroupByClause  :: !GroupByClause
                             , sdHavingClause   :: !HavingClause
                             , sdOrderByClause  :: ![OrderByClause]
                             , sdLimitClause    :: !LimitClause
                             , sdLockingClause  :: !LockingClause
                             }

    instance Monoid SideData where
      mempty = SideData mempty mempty mempty mempty mempty mempty mempty mempty mempty
      SideData d f s w g h o l k `mappend` SideData d' f' s' w' g' h' o' l' k' =
        SideData (d <> d') (f <> f') (s <> s') (w <> w') (g <> g') (h <> h') (o <> o') (l <> l') (k <> k')

    -- | The @DISTINCT@ "clause".
    data DistinctClause =
        DistinctAll                     -- ^ The default, everything.
      | DistinctStandard                -- ^ Only @DISTINCT@, SQL standard.
      | DistinctOn [SqlExpr DistinctOn] -- ^ @DISTINCT ON@, PostgreSQL extension.

    instance Monoid DistinctClause where
      mempty = DistinctAll
      DistinctOn a     `mappend` DistinctOn b = DistinctOn (a <> b)
      DistinctOn a     `mappend` _            = DistinctOn a
      DistinctStandard `mappend` _            = DistinctStandard
      DistinctAll      `mappend` b            = b

    -- | A part of a @FROM@ clause.
    data FromClause =
        FromStart Ident EntityDef
      | FromJoin FromClause JoinKind FromClause (Maybe (SqlExpr (Value Bool)))
      | OnClause (SqlExpr (Value Bool))

    -- | A part of a @SET@ clause.
    newtype SetClause = SetClause (SqlExpr (Value ()))

    -- | Collect 'OnClause's on 'FromJoin's.  Returns the first
    -- unmatched 'OnClause's data on error.  Returns a list without
    -- 'OnClauses' on success.
    collectOnClauses :: [FromClause] -> Either (SqlExpr (Value Bool)) [FromClause]
    collectOnClauses = go []
      where
        go []  (f@(FromStart _ _):fs) = fmap (f:) (go [] fs) -- fast path
        go acc (OnClause expr    :fs) = findMatching acc expr >>= flip go fs
        go acc (f:fs)                 = go (f:acc) fs
        go acc []                     = return $ reverse acc

        findMatching (f : acc) expr =
          case tryMatch expr f of
            Just f' -> return (f' : acc)
            Nothing -> (f:) <$> findMatching acc expr
        findMatching [] expr = Left expr

        tryMatch expr (FromJoin l k r onClause) =
          matchR `mplus` matchC `mplus` matchL -- right to left
            where
              matchR = (\r' -> FromJoin l k r' onClause) <$> tryMatch expr r
              matchL = (\l' -> FromJoin l' k r onClause) <$> tryMatch expr l
              matchC = case onClause of
                         Nothing | k /= CrossJoinKind
                                   -> return (FromJoin l k r (Just expr))
                                 | otherwise -> mzero
                         Just _  -> mzero
        tryMatch _ _ = mzero

    -- | A complete @WHERE@ clause.
    data WhereClause = Where (SqlExpr (Value Bool))
                     | NoWhere

    instance Monoid WhereClause where
      mempty = NoWhere
      NoWhere  `mappend` w        = w
      w        `mappend` NoWhere  = w
      Where e1 `mappend` Where e2 = Where (e1 &&. e2)

    -- | A @GROUP BY@ clause.
    newtype GroupByClause = GroupBy [SomeValue SqlExpr]

    instance Monoid GroupByClause where
      mempty = GroupBy []
      GroupBy fs `mappend` GroupBy fs' = GroupBy (fs <> fs')

    -- | A @HAVING@ cause.
    type HavingClause = WhereClause

    -- | A @ORDER BY@ clause.
    type OrderByClause = SqlExpr OrderBy

    -- | A @LIMIT@ clause.
    data LimitClause = Limit (Maybe Int64) (Maybe Int64)

    instance Monoid LimitClause where
      mempty = Limit mzero mzero
      Limit l1 o1 `mappend` Limit l2 o2 =
        Limit (l2 `mplus` l1) (o2 `mplus` o1)
        -- More than one 'limit' or 'offset' is issued, we want to
        -- keep the latest one.  That's why we use mplus with
        -- "reversed" arguments.

    -- | A locking clause.
    type LockingClause = Last LockingKind

    ----------------------------------------------------------------------

    -- | Identifier used for table names.
    newtype Ident = I T.Text

    -- | List of identifiers already in use and supply of temporary
    -- identifiers.
    newtype IdentState = IdentState { inUse :: HS.HashSet T.Text }

    initialIdentState :: IdentState
    initialIdentState = IdentState mempty

    -- | Create a fresh 'Ident'.  If possible, use the given
    -- 'DBName'.
    newIdentFor :: DBName -> SqlQuery Ident
    newIdentFor = Q . lift . try . unDBName
      where
        try orig = do
          s <- S.get
          let go (t:ts) | t `HS.member` inUse s = go ts
                        | otherwise             = use t
              go [] = error "Esqueleto/Sql/newIdentFor: never here"
          go (possibilities orig)

        possibilities t = t : map addNum [2..]
          where
            addNum :: Int -> T.Text
            addNum = T.append t . T.pack . show

        use t = do
          S.modify (\s -> s { inUse = HS.insert t (inUse s) })
          return (I t)

    -- | Information needed to escape and use identifiers.
    type IdentInfo = (SqlBackend, IdentState)

    -- | Use an identifier.
    useIdent :: IdentInfo -> Ident -> TLB.Builder
    useIdent info (I ident) = fromDBName info $ DBName ident

    ----------------------------------------------------------------------

    -- | An expression on the SQL backend.
    --
    -- There are many comments describing the constructors of this
    -- data type.  However, Haddock doesn't like GADTs, so you'll have to read them by hitting \"Source\".
    data SqlExpr a where
      -- An entity, created by 'from' (cf. 'fromStart').
      EEntity  :: Ident -> SqlExpr (Entity val)

      -- Just a tag stating that something is nullable.
      EMaybe   :: SqlExpr a -> SqlExpr (Maybe a)

      -- Raw expression: states whether parenthesis are needed
      -- around this expression, and takes information about the SQL
      -- connection (mainly for escaping names) and returns both an
      -- string ('TLB.Builder') and a list of values to be
      -- interpolated by the SQL backend.
      ERaw     :: NeedParens -> (IdentInfo -> (TLB.Builder, [PersistValue])) -> SqlExpr (Value a)

      -- A composite key.
      --
      -- Persistent uses the same 'PersistList' constructor for both
      -- fields which are (homogeneous) lists of values and the
      -- (probably heterogeneous) values of a composite primary key.
      --
      -- We need to treat composite keys as fields.  For example, we
      -- have to support using ==., otherwise you wouldn't be able to
      -- join.  OTOH, lists of values should be treated exactly the
      -- same as any other scalar value.
      --
      -- In particular, this is valid for persistent via rawSql for
      -- an F field that is a list:
      --
      --   A.F in ?    -- [PersistList [foo, bar]]
      --
      -- However, this is not for a composite key entity:
      --
      --   A.ID = ?    -- [PersistList [foo, bar]]
      --
      -- The ID field doesn't exist on the DB for a composite key
      -- table, it exists only on the Haskell side.  Those variations
      -- also don't work:
      --
      --   (A.KeyA, A.KeyB) = ?    -- [PersistList [foo, bar]]
      --   [A.KeyA, A.KeyB] = ?    -- [PersistList [foo, bar]]
      --
      -- We have to generate:
      --
      --   A.KeyA = ? AND A.KeyB = ?      -- [foo, bar]
      --
      -- Note that the PersistList had to be deconstructed into its
      -- components.
      --
      -- In order to disambiguate behaviors, this constructor is used
      -- /only/ to represent a composite field access.  It does not
      -- represent a 'PersistList', not even if the 'PersistList' is
      -- used in the context of a composite key.  That's because it's
      -- impossible, e.g., for 'val' to disambiguate between these
      -- uses.
      ECompositeKey :: (IdentInfo -> [TLB.Builder]) -> SqlExpr (Value a)

      -- 'EList' and 'EEmptyList' are used by list operators.
      EList      :: SqlExpr (Value a) -> SqlExpr (ValueList a)
      EEmptyList :: SqlExpr (ValueList a)

      -- A 'SqlExpr' accepted only by 'orderBy'.
      EOrderBy :: OrderByType -> SqlExpr (Value a) -> SqlExpr OrderBy
      EOrderRandom :: SqlExpr OrderBy

      -- A 'SqlExpr' accepted only by 'distinctOn'.
      EDistinctOn :: SqlExpr (Value a) -> SqlExpr DistinctOn

      -- A 'SqlExpr' accepted only by 'set'.
      ESet :: (SqlExpr (Entity val) -> SqlExpr (Value ())) -> SqlExpr (Update val)

      -- An internal 'SqlExpr' used by the 'from' hack.
      EPreprocessedFrom :: a -> FromClause -> SqlExpr (PreprocessedFrom a)

      -- Used by 'insertSelect'.
      EInsert  :: Proxy a -> (IdentInfo -> (TLB.Builder, [PersistValue])) -> SqlExpr (Insertion a)
      EInsertFinal :: PersistEntity a => SqlExpr (Insertion a) -> SqlExpr InsertFinal

    -- | Phantom type used to mark a @INSERT INTO@ query.
    data InsertFinal

    data NeedParens = Parens | Never

    parensM :: NeedParens -> TLB.Builder -> TLB.Builder
    parensM Never  = id
    parensM Parens = parens

    data OrderByType = ASC | DESC

    instance Esqueleto SqlQuery SqlExpr SqlBackend where
      fromStart = x
        where
          x = do
            let ed = entityDef (getVal x)
            ident <- newIdentFor (entityDB ed)
            let ret   = EEntity ident
                from_ = FromStart ident ed
            return (EPreprocessedFrom ret from_)
          getVal :: SqlQuery (SqlExpr (PreprocessedFrom (SqlExpr (Entity a)))) -> Proxy a
          getVal = const Proxy

      fromStartMaybe = maybelize <$> fromStart
        where
          maybelize :: SqlExpr (PreprocessedFrom (SqlExpr (Entity a)))
                    -> SqlExpr (PreprocessedFrom (SqlExpr (Maybe (Entity a))))
          maybelize (EPreprocessedFrom ret from_) = EPreprocessedFrom (EMaybe ret) from_

      fromJoin (EPreprocessedFrom lhsRet lhsFrom)
               (EPreprocessedFrom rhsRet rhsFrom) = Q $ do
        let ret   = smartJoin lhsRet rhsRet
            from_ = FromJoin lhsFrom             -- LHS
                             (reifyJoinKind ret) -- JOIN
                             rhsFrom             -- RHS
                             Nothing             -- ON
        return (EPreprocessedFrom ret from_)

      fromFinish (EPreprocessedFrom ret from_) = Q $ do
        W.tell mempty { sdFromClause = [from_] }
        return ret

      where_ expr = Q $ W.tell mempty { sdWhereClause = Where expr }

      on expr = Q $ W.tell mempty { sdFromClause = [OnClause expr] }

      groupBy expr = Q $ W.tell mempty { sdGroupByClause = GroupBy $ toSomeValues expr }

      having expr = Q $ W.tell mempty { sdHavingClause = Where expr }

      locking kind = Q $ W.tell mempty { sdLockingClause = Last (Just kind) }

      orderBy exprs = Q $ W.tell mempty { sdOrderByClause = exprs }
      asc  = EOrderBy ASC
      desc = EOrderBy DESC

      rand = EOrderRandom

      limit  n = Q $ W.tell mempty { sdLimitClause = Limit (Just n) Nothing  }
      offset n = Q $ W.tell mempty { sdLimitClause = Limit Nothing  (Just n) }

      distinct         act = Q (W.tell mempty { sdDistinctClause = DistinctStandard }) >> act
      distinctOn exprs act = Q (W.tell mempty { sdDistinctClause = DistinctOn exprs }) >> act
      don = EDistinctOn
      distinctOnOrderBy exprs act =
        distinctOn (toDistinctOn <$> exprs) $ do
          orderBy exprs
          act
        where
          toDistinctOn :: SqlExpr OrderBy -> SqlExpr DistinctOn
          toDistinctOn (EOrderBy _ f) = EDistinctOn f

      sub_select         = sub SELECT
      sub_selectDistinct = sub_select . distinct

      (^.) :: forall val typ. (PersistEntity val, PersistField typ)
           => SqlExpr (Entity val) -> EntityField val typ -> SqlExpr (Value typ)
      EEntity ident ^. field
        | isComposite = ECompositeKey $ \info ->  dot info <$> compositeFields pdef
        | otherwise   = ERaw Never    $ \info -> (dot info  $  persistFieldDef field, [])
        where
          isComposite = isIdField field && hasCompositeKey ed
          dot info x  = useIdent info ident <> "." <> fromDBName info (fieldDB x)
          ed          = entityDef $ getEntityVal (Proxy :: Proxy (SqlExpr (Entity val)))
          Just pdef   = entityPrimary ed

      EMaybe r ?. field = just (r ^. field)

      val v = ERaw Never $ const ("?", [toPersistValue v])

      isNothing (ERaw p f)        = ERaw Parens $ first ((<> " IS NULL") . parensM p) . f
      isNothing (ECompositeKey f) = ERaw Parens $ flip (,) [] . (intersperseB " AND " . map (<> " IS NULL")) . f
      just (ERaw p f)        = ERaw p f
      just (ECompositeKey f) = ECompositeKey f
      nothing = unsafeSqlValue "NULL"
      joinV (ERaw p f)        = ERaw p f
      joinV (ECompositeKey f) = ECompositeKey f
      countRows     = unsafeSqlValue "COUNT(*)"
      count         = countHelper ""           ""
      countDistinct = countHelper "(DISTINCT " ")"

      not_ (ERaw p f) = ERaw Never $ \info -> let (b, vals) = f info
                                              in ("NOT " <> parensM p b, vals)
      not_ (ECompositeKey _) = unexpectedCompositeKeyError "not_"

      (==.) = unsafeSqlBinOpComposite " = " " AND "
      (!=.) = unsafeSqlBinOpComposite " != " " OR "
      (>=.) = unsafeSqlBinOp " >= "
      (>.)  = unsafeSqlBinOp " > "
      (<=.) = unsafeSqlBinOp " <= "
      (<.)  = unsafeSqlBinOp " < "
      (&&.) = unsafeSqlBinOp " AND "
      (||.) = unsafeSqlBinOp " OR "
      (+.)  = unsafeSqlBinOp " + "
      (-.)  = unsafeSqlBinOp " - "
      (/.)  = unsafeSqlBinOp " / "
      (*.)  = unsafeSqlBinOp " * "

      random_  = unsafeSqlValue "RANDOM()"
      round_   = unsafeSqlFunction "ROUND"
      ceiling_ = unsafeSqlFunction "CEILING"
      floor_   = unsafeSqlFunction "FLOOR"

      sum_     = unsafeSqlFunction "SUM"
      min_     = unsafeSqlFunction "MIN"
      max_     = unsafeSqlFunction "MAX"
      avg_     = unsafeSqlFunction "AVG"

      castNum  = veryUnsafeCoerceSqlExprValue
      castNumM = veryUnsafeCoerceSqlExprValue

      coalesce              = unsafeSqlFunctionParens "COALESCE"
      coalesceDefault exprs = unsafeSqlFunctionParens "COALESCE" . (exprs ++) . return . just

      lower_  = unsafeSqlFunction "LOWER"
      like    = unsafeSqlBinOp    " LIKE "
      ilike   = unsafeSqlBinOp    " ILIKE "
      (%)     = unsafeSqlValue    "'%'"
      concat_ = unsafeSqlFunction "CONCAT"
      (++.)   = unsafeSqlBinOp    " || "
      castString = veryUnsafeCoerceSqlExprValue

      subList_select         = EList . sub_select
      subList_selectDistinct = subList_select . distinct

      valList []   = EEmptyList
      valList vals = EList $ ERaw Parens $ const ( uncommas ("?" <$ vals)
                                                 , map toPersistValue vals )

      justList EEmptyList = EEmptyList
      justList (EList v)  = EList (just v)

      v `in_`   e = ifNotEmptyList e False $ unsafeSqlBinOp     " IN " v (veryUnsafeCoerceSqlExprValueList e)
      v `notIn` e = ifNotEmptyList e True  $ unsafeSqlBinOp " NOT IN " v (veryUnsafeCoerceSqlExprValueList e)

      exists    = unsafeSqlFunction     "EXISTS " . existsHelper
      notExists = unsafeSqlFunction "NOT EXISTS " . existsHelper

      set ent upds = Q $ W.tell mempty { sdSetClause = map apply upds }
        where
          apply (ESet f) = SetClause (f ent)

      field  =. expr = setAux field (const expr)
      field +=. expr = setAux field (\ent -> ent ^. field +. expr)
      field -=. expr = setAux field (\ent -> ent ^. field -. expr)
      field *=. expr = setAux field (\ent -> ent ^. field *. expr)
      field /=. expr = setAux field (\ent -> ent ^. field /. expr)

      (<#) _ (ERaw _ f)        = EInsert Proxy f
      (<#) _ (ECompositeKey _) = unexpectedCompositeKeyError "(<#)"

      (EInsert _ f) <&> (ERaw _ g) = EInsert Proxy $ \x ->
        let (fb, fv) = f x
            (gb, gv) = g x
        in (fb <> ", " <> gb, fv ++ gv)
      (EInsert _ _) <&> (ECompositeKey _) = unexpectedCompositeKeyError "(<&>)"

      case_ = unsafeSqlCase

    instance ToSomeValues SqlExpr (SqlExpr (Value a)) where
      toSomeValues a = [SomeValue a]

    fieldName :: (PersistEntity val, PersistField typ)
              => IdentInfo -> EntityField val typ -> TLB.Builder
    fieldName info = fromDBName info . fieldDB . persistFieldDef

    -- FIXME: Composite/non-id pKS not supported on set
    setAux :: (PersistEntity val, PersistField typ)
           => EntityField val typ
           -> (SqlExpr (Entity val) -> SqlExpr (Value typ))
           -> SqlExpr (Update val)
    setAux field mkVal = ESet $ \ent -> unsafeSqlBinOp " = " name (mkVal ent)
      where name = ERaw Never $ \info -> (fieldName info field, mempty)

    sub :: PersistField a => Mode -> SqlQuery (SqlExpr (Value a)) -> SqlExpr (Value a)
    sub mode query = ERaw Parens $ \info -> toRawSql mode info query

    fromDBName :: IdentInfo -> DBName -> TLB.Builder
    fromDBName (conn, _) = TLB.fromText . connEscapeName conn

    existsHelper :: SqlQuery () -> SqlExpr (Value Bool)
    existsHelper = sub SELECT . (>> return true)
      where
        true :: SqlExpr (Value Bool)
        true = val True

    ifNotEmptyList :: SqlExpr (ValueList a) -> Bool -> SqlExpr (Value Bool) -> SqlExpr (Value Bool)
    ifNotEmptyList EEmptyList b _ = val b
    ifNotEmptyList (EList _)  _ x = x

    countHelper :: Num a => TLB.Builder -> TLB.Builder -> SqlExpr (Value typ) -> SqlExpr (Value a)
    countHelper open close (ERaw _ f) = ERaw Never $ first (\b -> "COUNT" <> open <> parens b <> close) . f
    countHelper _ _ (ECompositeKey _) = countRows -- Assumes no NULLs on a PK

    ----------------------------------------------------------------------

    -- | (Internal) Create a case statement.
    --
    -- Since: 2.1.1
    unsafeSqlCase :: PersistField a => [(SqlExpr (Value Bool), SqlExpr (Value a))] -> SqlExpr (Value a) -> SqlExpr (Value a)
    unsafeSqlCase when (ERaw p1 f1) = ERaw Never buildCase
      where
        buildCase :: IdentInfo -> (TLB.Builder, [PersistValue])
        buildCase info =
            let (b1, vals1) = f1 info
                (b2, vals2) = mapWhen when info
            in ( "CASE" <> b2 <> " ELSE " <> parensM p1 b1 <> " END", vals2 <> vals1)

        mapWhen :: [(SqlExpr (Value Bool), SqlExpr (Value a))] -> IdentInfo -> (TLB.Builder, [PersistValue])
        mapWhen []    _    = error "unsafeSqlCase: empty when list."
        mapWhen when' info = foldl (foldHelp info) (mempty, mempty) when'

        foldHelp :: IdentInfo -> (TLB.Builder, [PersistValue]) -> (SqlExpr (Value Bool), SqlExpr (Value a)) -> (TLB.Builder, [PersistValue])
        foldHelp info (b0, vals0) (ERaw p1' f1', ERaw p2 f2) =
            let (b1, vals1) = f1' info
                (b2, vals2) = f2 info
            in ( b0 <> " WHEN " <> parensM p1' b1 <> " THEN " <> parensM p2 b2, vals0 <> vals1 <> vals2 )
        foldHelp _ _ _ = unexpectedCompositeKeyError "unsafeSqlCase/foldHelp"
    unsafeSqlCase _ (ECompositeKey _) = unexpectedCompositeKeyError "unsafeSqlCase"

    -- | (Internal) Create a custom binary operator.  You /should/
    -- /not/ use this function directly since its type is very
    -- general, you should always use it with an explicit type
    -- signature.  For example:
    --
    -- @
    -- (==.) :: SqlExpr (Value a) -> SqlExpr (Value a) -> SqlExpr (Value Bool)
    -- (==.) = unsafeSqlBinOp " = "
    -- @
    --
    -- In the example above, we constraint the arguments to be of the
    -- same type and constraint the result to be a boolean value.
    unsafeSqlBinOp :: TLB.Builder -> SqlExpr (Value a) -> SqlExpr (Value b) -> SqlExpr (Value c)
    unsafeSqlBinOp op (ERaw p1 f1) (ERaw p2 f2) = ERaw Parens f
      where
        f info = let (b1, vals1) = f1 info
                     (b2, vals2) = f2 info
                 in ( parensM p1 b1 <> op <> parensM p2 b2
                    , vals1 <> vals2 )
    unsafeSqlBinOp _ _ _ = unexpectedCompositeKeyError "unsafeSqlBinOp"
    {-# INLINE unsafeSqlBinOp #-}

    -- | Similar to 'unsafeSqlBinOp', but may also be applied to
    -- composite keys.  Uses the operator given as the second
    -- argument whenever applied to composite keys.
    --
    -- Usage example:
    --
    -- @
    -- (==.) :: SqlExpr (Value a) -> SqlExpr (Value a) -> SqlExpr (Value Bool)
    -- (==.) = unsafeSqlBinOpComposite " = " " AND "
    -- @
    --
    -- Persistent has a hack for implementing composite keys (see
    -- 'ECompositeKey' doc for more details), so we're forced to use
    -- a hack here as well.  We deconstruct 'ERaw' values based on
    -- two rules:
    --
    --   - If it is a single placeholder, then it's assumed to be
    --   coming from a 'PersistList' and thus its components are
    --   separated so that they may be applied to a composite key.
    --
    --   - If it is not a single placeholder, then it's assumed to be
    --   a foreign (composite or not) key, so we enforce that it has
    --   no placeholders and split it on the commas.
    unsafeSqlBinOpComposite :: TLB.Builder -> TLB.Builder -> SqlExpr (Value a) -> SqlExpr (Value b) -> SqlExpr (Value c)
    unsafeSqlBinOpComposite op _ a@(ERaw _ _) b@(ERaw _ _) = unsafeSqlBinOp op a b
    unsafeSqlBinOpComposite op sep a b = ERaw Parens $ compose (listify a) (listify b)
      where
        listify :: SqlExpr (Value x) -> IdentInfo -> ([TLB.Builder], [PersistValue])
        listify (ECompositeKey f) = flip (,) [] . f
        listify (ERaw _ f)        = deconstruct . f

        deconstruct :: (TLB.Builder, [PersistValue]) -> ([TLB.Builder], [PersistValue])
        deconstruct ("?", [PersistList vals]) = (replicate (length vals) "?", vals)
        deconstruct (b, []) = (TLB.fromLazyText <$> TL.splitOn "," (TLB.toLazyText b), [])
        deconstruct x = err $ "cannot deconstruct " ++ show x ++ "."

        compose f1 f2 info
          | not (null v1 || null v2) = err' "one side needs to have null placeholders"
          | length b1 /= length b2   = err' "mismatching lengths"
          | otherwise                = (bc, vc)
          where
            (b1, v1) = f1 info
            (b2, v2) = f2 info
            bc = intersperseB sep [x <> op <> y | (x, y) <- zip b1 b2]
            vc = v1 <> v2
            err' = err . (++ (", " ++ show ((b1, v1), (b2, v2))))

        err = error . ("unsafeSqlBinOpComposite: " ++)

    -- | (Internal) A raw SQL value.  The same warning from
    -- 'unsafeSqlBinOp' applies to this function as well.
    unsafeSqlValue :: TLB.Builder -> SqlExpr (Value a)
    unsafeSqlValue v = ERaw Never $ \_ -> (v, mempty)
    {-# INLINE unsafeSqlValue #-}

    -- | (Internal) A raw SQL function.  Once again, the same warning
    -- from 'unsafeSqlBinOp' applies to this function as well.
    unsafeSqlFunction :: UnsafeSqlFunctionArgument a =>
                         TLB.Builder -> a -> SqlExpr (Value b)
    unsafeSqlFunction name arg =
      ERaw Never $ \info ->
        let (argsTLB, argsVals) =
              uncommas' $ map (\(ERaw _ f) -> f info) $ toArgList arg
        in (name <> parens argsTLB, argsVals)

    -- | (Internal) An unsafe SQL function to extract a subfield from a compound
    -- field, e.g. datetime. See 'unsafeSqlBinOp' for warnings.
    --
    -- Since: 1.3.6.
    unsafeSqlExtractSubField :: UnsafeSqlFunctionArgument a =>
                         TLB.Builder -> a -> SqlExpr (Value b)
    unsafeSqlExtractSubField subField arg =
      ERaw Never $ \info ->
        let (argsTLB, argsVals) =
              uncommas' $ map (\(ERaw _ f) -> f info) $ toArgList arg
        in ("EXTRACT" <> parens (subField <> " FROM " <> argsTLB), argsVals)

    -- | (Internal) A raw SQL function. Preserves parentheses around arguments.
    -- See 'unsafeSqlBinOp' for warnings.
    unsafeSqlFunctionParens :: UnsafeSqlFunctionArgument a =>
                               TLB.Builder -> a -> SqlExpr (Value b)
    unsafeSqlFunctionParens name arg =
      ERaw Never $ \info ->
        let (argsTLB, argsVals) =
              uncommas' $ map (\(ERaw p f) -> first (parensM p) (f info)) $ toArgList arg
        in (name <> parens argsTLB, argsVals)

    class UnsafeSqlFunctionArgument a where
      toArgList :: a -> [SqlExpr (Value ())]
    instance (a ~ Value b) => UnsafeSqlFunctionArgument (SqlExpr a) where
      toArgList = (:[]) . veryUnsafeCoerceSqlExprValue
    instance UnsafeSqlFunctionArgument a =>
             UnsafeSqlFunctionArgument [a] where
      toArgList = concatMap toArgList
    instance ( UnsafeSqlFunctionArgument a
             , UnsafeSqlFunctionArgument b
             ) => UnsafeSqlFunctionArgument (a, b) where
      toArgList (a, b) = toArgList a ++ toArgList b
    instance ( UnsafeSqlFunctionArgument a
             , UnsafeSqlFunctionArgument b
             , UnsafeSqlFunctionArgument c
             ) => UnsafeSqlFunctionArgument (a, b, c) where
      toArgList = toArgList . from3
    instance ( UnsafeSqlFunctionArgument a
             , UnsafeSqlFunctionArgument b
             , UnsafeSqlFunctionArgument c
             , UnsafeSqlFunctionArgument d
             ) => UnsafeSqlFunctionArgument (a, b, c, d) where
      toArgList = toArgList . from4

    -- | (Internal) Coerce a value's type from 'SqlExpr (Value a)' to
    -- 'SqlExpr (Value b)'.  You should /not/ use this function
    -- unless you know what you're doing!
    veryUnsafeCoerceSqlExprValue :: SqlExpr (Value a) -> SqlExpr (Value b)
    veryUnsafeCoerceSqlExprValue (ERaw p f)        = ERaw p f
    veryUnsafeCoerceSqlExprValue (ECompositeKey f) = ECompositeKey f

    -- | (Internal) Coerce a value's type from 'SqlExpr (ValueList
    -- a)' to 'SqlExpr (Value a)'.  Does not work with empty lists.
    veryUnsafeCoerceSqlExprValueList :: SqlExpr (ValueList a) -> SqlExpr (Value a)
    veryUnsafeCoerceSqlExprValueList (EList v)  = v
    veryUnsafeCoerceSqlExprValueList EEmptyList =
      error "veryUnsafeCoerceSqlExprValueList: empty list."

    ----------------------------------------------------------------------

    -- | (Internal) Execute an @esqueleto@ @SELECT@ 'SqlQuery' inside
    -- @persistent@'s 'SqlPersistT' monad.
    rawSelectSource :: ( SqlSelect a r
                       , MonadIO m1
                       , MonadIO m2 )
                     => Mode
                     -> SqlQuery a
                     -> SqlPersistT m1 (Acquire (C.Source m2 r))
    rawSelectSource mode query =
          do
            conn <- R.ask
            res <- run conn
            return $ (C.$= massage) `fmap` res
        where

          run conn =
            uncurry rawQueryRes $
            first builderToText $
            toRawSql mode (conn, initialIdentState) query

          massage = do
            mrow <- C.await
            case process <$> mrow of
              Just (Right r)  -> C.yield r >> massage
              Just (Left err) -> liftIO $ throwIO $ PersistMarshalError err
              Nothing         -> return ()

          process = sqlSelectProcessRow

    -- | Execute an @esqueleto@ @SELECT@ query inside @persistent@'s
    -- 'SqlPersistT' monad and return a 'C.Source' of rows.
    selectSource :: ( SqlSelect a r
                    , MonadResource m )
                 => SqlQuery a
                 -> C.Source (SqlPersistT m) r
    selectSource query = do
        src <- lift $ do
            res <- rawSelectSource SELECT query
            fmap snd $ allocateAcquire res
        src

    -- | Execute an @esqueleto@ @SELECT@ query inside @persistent@'s
    -- 'SqlPersistT' monad and return a list of rows.
    --
    -- We've seen that 'from' has some magic about which kinds of
    -- things you may bring into scope.  This 'select' function also
    -- has some magic for which kinds of things you may bring back to
    -- Haskell-land by using @SqlQuery@'s @return@:
    --
    --  * You may return a @SqlExpr ('Entity' v)@ for an entity @v@
    --  (i.e., like the @*@ in SQL), which is then returned to
    --  Haskell-land as just @Entity v@.
    --
    --  * You may return a @SqlExpr (Maybe (Entity v))@ for an entity
    --  @v@ that may be @NULL@, which is then returned to
    --  Haskell-land as @Maybe (Entity v)@.  Used for @OUTER JOIN@s.
    --
    --  * You may return a @SqlExpr ('Value' t)@ for a value @t@
    --  (i.e., a single column), where @t@ is any instance of
    --  'PersistField', which is then returned to Haskell-land as
    --  @Value t@.  You may use @Value@ to return projections of an
    --  @Entity@ (see @('^.')@ and @('?.')@) or to return any other
    --  value calculated on the query (e.g., 'countRows' or
    --  'sub_select').
    --
    -- The @SqlSelect a r@ class has functional dependencies that
    -- allow type information to flow both from @a@ to @r@ and
    -- vice-versa.  This means that you'll almost never have to give
    -- any type signatures for @esqueleto@ queries.  For example, the
    -- query @'select' $ from $ \\p -> return p@ alone is ambiguous, but
    -- in the context of
    --
    -- @
    -- do ps <- 'select' $
    --          'from' $ \\p ->
    --          return p
    --    liftIO $ mapM_ (putStrLn . personName . entityVal) ps
    -- @
    --
    -- we are able to infer from that single @personName . entityVal@
    -- function composition that the @p@ inside the query is of type
    -- @SqlExpr (Entity Person)@.
    select :: ( SqlSelect a r
              , MonadIO m )
           => SqlQuery a -> SqlPersistT m [r]
    select query = do
        res <- rawSelectSource SELECT query
        conn <- R.ask
        liftIO $ with res $ flip R.runReaderT conn . runSource

    -- | Execute an @esqueleto@ @SELECT DISTINCT@ query inside
    -- @persistent@'s 'SqlPersistT' monad and return a 'C.Source' of
    -- rows.
    selectDistinctSource
      :: ( SqlSelect a r
         , MonadResource m )
      => SqlQuery a
      -> C.Source (SqlPersistT m) r
    selectDistinctSource = selectSource . distinct
    {-# DEPRECATED selectDistinctSource "Since 2.2.4: use 'selectSource' and 'distinct'." #-}

    -- | Execute an @esqueleto@ @SELECT DISTINCT@ query inside
    -- @persistent@'s 'SqlPersistT' monad and return a list of rows.
    selectDistinct :: ( SqlSelect a r
                      , MonadIO m )
                   => SqlQuery a -> SqlPersistT m [r]
    selectDistinct = select . distinct
    {-# DEPRECATED selectDistinct "Since 2.2.4: use 'select' and 'distinct'." #-}

    -- | (Internal) Run a 'C.Source' of rows.
    runSource :: Monad m =>
                 C.Source (SqlPersistT m) r
              -> SqlPersistT m [r]
    runSource src = src C.$$ CL.consume

    ----------------------------------------------------------------------

    -- | (Internal) Execute an @esqueleto@ statement inside
    -- @persistent@'s 'SqlPersistT' monad.
    rawEsqueleto :: ( MonadIO m, SqlSelect a r )
               => Mode
               -> SqlQuery a
               -> SqlPersistT m Int64
    rawEsqueleto mode query = do
      conn <- R.ask
      uncurry rawExecuteCount $
        first builderToText $
        toRawSql mode (conn, initialIdentState) query

    -- | Execute an @esqueleto@ @DELETE@ query inside @persistent@'s
    -- 'SqlPersistT' monad.  Note that currently there are no type
    -- checks for statements that should not appear on a @DELETE@
    -- query.
    --
    -- Example of usage:
    --
    -- @
    -- 'delete' $
    -- 'from' $ \\appointment ->
    -- 'where_' (appointment '^.' AppointmentDate '<.' 'val' now)
    -- @
    --
    -- Unlike 'select', there is a useful way of using 'delete' that
    -- will lead to type ambiguities.  If you want to delete all rows
    -- (i.e., no 'where_' clause), you'll have to use a type signature:
    --
    -- @
    -- 'delete' $
    -- 'from' $ \\(appointment :: 'SqlExpr' ('Entity' Appointment)) ->
    -- return ()
    -- @
    delete :: ( MonadIO m )
           => SqlQuery ()
           -> SqlPersistT m ()
    delete = liftM (const ()) . deleteCount

    -- | Same as 'delete', but returns the number of rows affected.
    deleteCount :: ( MonadIO m )
                => SqlQuery ()
                -> SqlPersistT m Int64
    deleteCount = rawEsqueleto DELETE

    -- | Execute an @esqueleto@ @UPDATE@ query inside @persistent@'s
    -- 'SqlPersistT' monad.  Note that currently there are no type
    -- checks for statements that should not appear on a @UPDATE@
    -- query.
    --
    -- Example of usage:
    --
    -- @
    -- 'update' $ \p -> do
    -- 'set' p [ PersonAge '=.' 'just' ('val' thisYear) -. p '^.' PersonBorn ]
    -- 'where_' $ isNothing (p '^.' PersonAge)
    -- @
    update :: ( MonadIO m
              , SqlEntity val )
           => (SqlExpr (Entity val) -> SqlQuery ())
           -> SqlPersistT m ()
    update = liftM (const ()) . updateCount

    -- | Same as 'update', but returns the number of rows affected.
    updateCount :: ( MonadIO m
                   , SqlEntity val )
                => (SqlExpr (Entity val) -> SqlQuery ())
                -> SqlPersistT m Int64
    updateCount = rawEsqueleto UPDATE . from

    ----------------------------------------------------------------------

    builderToText :: TLB.Builder -> T.Text
    builderToText = TL.toStrict . TLB.toLazyTextWith defaultChunkSize
      where
        defaultChunkSize = 1024 - 32

    -- | (Internal) Pretty prints a 'SqlQuery' into a SQL query.
    --
    -- Note: if you're curious about the SQL query being generated by
    -- @esqueleto@, instead of manually using this function (which is
    -- possible but tedious), you may just turn on query logging of
    -- @persistent@.
    toRawSql :: SqlSelect a r => Mode -> IdentInfo -> SqlQuery a -> (TLB.Builder, [PersistValue])
    toRawSql mode (conn, firstIdentState) query =
      let ((ret, sd), finalIdentState) =
            flip S.runState firstIdentState $
            W.runWriterT $
            unQ query
          SideData distinctClause
                   fromClauses
                   setClauses
                   whereClauses
                   groupByClause
                   havingClause
                   orderByClauses
                   limitClause
                   lockingClause = sd
          -- Pass the finalIdentState (containing all identifiers
          -- that were used) to the subsequent calls.  This ensures
          -- that no name clashes will occur on subqueries that may
          -- appear on the expressions below.
          info = (conn, finalIdentState)
      in mconcat
          [ makeInsertInto info mode ret
          , makeSelect     info mode distinctClause ret
          , makeFrom       info mode fromClauses
          , makeSet        info setClauses
          , makeWhere      info whereClauses
          , makeGroupBy    info groupByClause
          , makeHaving     info havingClause
          , makeOrderBy    info orderByClauses
          , makeLimit      info limitClause orderByClauses
          , makeLocking         lockingClause
          ]

    -- | (Internal) Mode of query being converted by 'toRawSql'.
    data Mode =
        SELECT
      | DELETE
      | UPDATE
      | INSERT_INTO

    uncommas :: [TLB.Builder] -> TLB.Builder
    uncommas = intersperseB ", "

    intersperseB :: TLB.Builder -> [TLB.Builder] -> TLB.Builder
    intersperseB a = mconcat . intersperse a . filter (/= mempty)

    uncommas' :: Monoid a => [(TLB.Builder, a)] -> (TLB.Builder, a)
    uncommas' = (uncommas *** mconcat) . unzip

    makeInsertInto :: SqlSelect a r => IdentInfo -> Mode -> a -> (TLB.Builder, [PersistValue])
    makeInsertInto info INSERT_INTO ret = sqlInsertInto info ret
    makeInsertInto _    _           _   = mempty

    makeSelect :: SqlSelect a r => IdentInfo -> Mode -> DistinctClause -> a -> (TLB.Builder, [PersistValue])
    makeSelect info mode_ distinctClause ret = process mode_
      where
        process mode =
          case mode of
            SELECT      -> withCols selectKind
            DELETE      -> plain "DELETE "
            UPDATE      -> plain "UPDATE "
            INSERT_INTO -> process SELECT
        selectKind =
          case distinctClause of
            DistinctAll      -> ("SELECT ", [])
            DistinctStandard -> ("SELECT DISTINCT ", [])
            DistinctOn exprs -> first (("SELECT DISTINCT ON (" <>) . (<> ") ")) $
                                uncommas' (processExpr <$> exprs)
          where processExpr (EDistinctOn f) = materializeExpr info f
        withCols v = v <> (sqlSelectCols info ret)
        plain    v = (v, [])

    makeFrom :: IdentInfo -> Mode -> [FromClause] -> (TLB.Builder, [PersistValue])
    makeFrom _    _    [] = mempty
    makeFrom info mode fs = ret
      where
        ret = case collectOnClauses fs of
                Left expr -> throw $ mkExc expr
                Right fs' -> keyword $ uncommas' (map (mk Never) fs')
        keyword = case mode of
                    UPDATE -> id
                    _      -> first ("\nFROM " <>)

        mk _     (FromStart i def) = base i def
        mk paren (FromJoin lhs kind rhs monClause) =
          first (parensM paren) $
          mconcat [ mk Never lhs
                  , (fromKind kind, mempty)
                  , mk Parens rhs
                  , maybe mempty makeOnClause monClause
                  ]
        mk _ (OnClause _) = error "Esqueleto/Sql/makeFrom: never here (is collectOnClauses working?)"

        base ident@(I identText) def =
          let db@(DBName dbText) = entityDB def
          in ( if dbText == identText
               then fromDBName info db
               else fromDBName info db <> (" AS " <> useIdent info ident)
             , mempty )

        fromKind InnerJoinKind      = " INNER JOIN "
        fromKind CrossJoinKind      = " CROSS JOIN "
        fromKind LeftOuterJoinKind  = " LEFT OUTER JOIN "
        fromKind RightOuterJoinKind = " RIGHT OUTER JOIN "
        fromKind FullOuterJoinKind  = " FULL OUTER JOIN "

        makeOnClause (ERaw _ f)        = first (" ON " <>) (f info)
        makeOnClause (ECompositeKey _) = unexpectedCompositeKeyError "makeFrom/makeOnClause"

        mkExc :: SqlExpr (Value Bool) -> OnClauseWithoutMatchingJoinException
        mkExc (ERaw _ f) =
          OnClauseWithoutMatchingJoinException $
          TL.unpack $ TLB.toLazyText $ fst (f info)
        mkExc (ECompositeKey _) = unexpectedCompositeKeyError "makeFrom/mkExc"

    unexpectedCompositeKeyError :: String -> a
    unexpectedCompositeKeyError w = error $ w ++ ": non-id/composite keys not expected here"

    makeSet :: IdentInfo -> [SetClause] -> (TLB.Builder, [PersistValue])
    makeSet _    [] = mempty
    makeSet info os = first ("\nSET " <>) . uncommas' $ concatMap mk os
      where
        mk (SetClause (ERaw _ f))        = [f info]
        mk (SetClause (ECompositeKey _)) = unexpectedCompositeKeyError "makeSet" -- FIXME

    makeWhere :: IdentInfo -> WhereClause -> (TLB.Builder, [PersistValue])
    makeWhere _    NoWhere                   = mempty
    makeWhere info (Where (ERaw _ f))        = first ("\nWHERE " <>) (f info)
    makeWhere _    (Where (ECompositeKey _)) = unexpectedCompositeKeyError "makeWhere"

    makeGroupBy :: IdentInfo -> GroupByClause -> (TLB.Builder, [PersistValue])
    makeGroupBy _ (GroupBy []) = (mempty, [])
    makeGroupBy info (GroupBy fields) = first ("\nGROUP BY " <>) build
      where
        build = uncommas' $ map (\(SomeValue (ERaw _ f)) -> f info) fields

    makeHaving :: IdentInfo -> WhereClause -> (TLB.Builder, [PersistValue])
    makeHaving _    NoWhere                    = mempty
    makeHaving info (Where (ERaw _ f))         = first ("\nHAVING " <>) (f info)
    makeHaving _    (Where (ECompositeKey _ )) = unexpectedCompositeKeyError "makeHaving"

    makeOrderBy :: IdentInfo -> [OrderByClause] -> (TLB.Builder, [PersistValue])
    makeOrderBy _    [] = mempty
    makeOrderBy info os = first ("\nORDER BY " <>) . uncommas' $ concatMap mk os
      where
        mk :: OrderByClause -> [(TLB.Builder, [PersistValue])]
        mk (EOrderBy t (ERaw p f)) = [first ((<> orderByType t) . parensM p) (f info)]
        mk (EOrderBy t (ECompositeKey f)) =
          let fs = f info
              vals = repeat []
          in zip (map (<> orderByType t) fs) vals
        mk EOrderRandom = [first ((<> "RANDOM()")) mempty]
        orderByType ASC  = " ASC"
        orderByType DESC = " DESC"

    makeLimit :: IdentInfo -> LimitClause -> [OrderByClause] -> (TLB.Builder, [PersistValue])
    makeLimit (conn, _) (Limit ml mo) orderByClauses =
      let limitRaw = connLimitOffset conn (v ml, v mo) hasOrderClause "\n"
          hasOrderClause = not (null orderByClauses)
          v = maybe 0 fromIntegral
      in (TLB.fromText limitRaw, mempty)

    makeLocking :: LockingClause -> (TLB.Builder, [PersistValue])
    makeLocking = flip (,) [] . maybe mempty toTLB . getLast
      where
        toTLB ForUpdate       = "\nFOR UPDATE"
        toTLB ForShare        = "\nFOR SHARE"
        toTLB LockInShareMode = "\nLOCK IN SHARE MODE"

    parens :: TLB.Builder -> TLB.Builder
    parens b = "(" <> (b <> ")")

    ----------------------------------------------------------------------

    -- | (Internal) Class for mapping results coming from 'SqlQuery'
    -- into actual results.
    --
    -- This looks very similar to @RawSql@, and it is!  However,
    -- there are some crucial differences and ultimately they're
    -- different classes.
    class SqlSelect a r | a -> r, r -> a where
      -- | Creates the variable part of the @SELECT@ query and
      -- returns the list of 'PersistValue's that will be given to
      -- 'rawQuery'.
      sqlSelectCols :: IdentInfo -> a -> (TLB.Builder, [PersistValue])

      -- | Number of columns that will be consumed.
      sqlSelectColCount :: Proxy a -> Int

      -- | Transform a row of the result into the data type.
      sqlSelectProcessRow :: [PersistValue] -> Either T.Text r

      -- | Create @INSERT INTO@ clause instead.
      sqlInsertInto :: IdentInfo -> a -> (TLB.Builder, [PersistValue])
      sqlInsertInto = error "Type does not support sqlInsertInto."

    -- | @INSERT INTO@ hack.
    instance SqlSelect (SqlExpr InsertFinal) InsertFinal where
      sqlInsertInto info (EInsertFinal (EInsert p _)) =
        let fields = uncommas $
                     map (fromDBName info . fieldDB) $
                     entityFields $
                     entityDef p
            table  = fromDBName info . entityDB . entityDef $ p
        in ("INSERT INTO " <> table <> parens fields <> "\n", [])
      sqlSelectCols info (EInsertFinal (EInsert _ f)) = f info
      sqlSelectColCount   = const 0
      sqlSelectProcessRow = const (Right (error msg))
        where
          msg = "sqlSelectProcessRow/SqlSelect/InsertionFinal: never here"

    -- | Not useful for 'select', but used for 'update' and 'delete'.
    instance SqlSelect () () where
      sqlSelectCols _ _ = ("1", [])
      sqlSelectColCount _ = 1
      sqlSelectProcessRow _ = Right ()

    -- | You may return an 'Entity' from a 'select' query.
    instance PersistEntity a => SqlSelect (SqlExpr (Entity a)) (Entity a) where
      sqlSelectCols info expr@(EEntity ident) = ret
          where
            process ed = uncommas $
                         map ((name <>) . TLB.fromText) $
                         entityColumnNames ed (fst info)
            -- 'name' is the biggest difference between 'RawSql' and
            -- 'SqlSelect'.  We automatically create names for tables
            -- (since it's not the user who's writing the FROM
            -- clause), while 'rawSql' assumes that it's just the
            -- name of the table (which doesn't allow self-joins, for
            -- example).
            name = useIdent info ident <> "."
            ret = let ed = entityDef $ getEntityVal $ return expr
                  in (process ed, mempty)
      sqlSelectColCount = entityColumnCount . entityDef . getEntityVal
      sqlSelectProcessRow = parseEntityValues ed
        where ed = entityDef $ getEntityVal $ (Proxy :: Proxy (SqlExpr (Entity a)))

    getEntityVal :: Proxy (SqlExpr (Entity a)) -> Proxy a
    getEntityVal = const Proxy

    -- | You may return a possibly-@NULL@ 'Entity' from a 'select' query.
    instance PersistEntity a => SqlSelect (SqlExpr (Maybe (Entity a))) (Maybe (Entity a)) where
      sqlSelectCols info (EMaybe ent) = sqlSelectCols info ent
      sqlSelectColCount = sqlSelectColCount . fromEMaybe
        where
          fromEMaybe :: Proxy (SqlExpr (Maybe e)) -> Proxy (SqlExpr e)
          fromEMaybe = const Proxy
      sqlSelectProcessRow cols
        | all (== PersistNull) cols = return Nothing
        | otherwise                 = Just <$> sqlSelectProcessRow cols

    -- | You may return any single value (i.e. a single column) from
    -- a 'select' query.
    instance PersistField a => SqlSelect (SqlExpr (Value a)) (Value a) where
      sqlSelectCols = materializeExpr
      sqlSelectColCount = const 1
      sqlSelectProcessRow [pv] = Value <$> fromPersistValue pv
      sqlSelectProcessRow pvs  = Value <$> fromPersistValue (PersistList pvs)

    -- | Materialize a @SqlExpr (Value a)@.
    materializeExpr :: IdentInfo -> SqlExpr (Value a) -> (TLB.Builder, [PersistValue])
    materializeExpr info (ERaw p f) =
      let (b, vals) = f info
      in (parensM p b, vals)
    materializeExpr info (ECompositeKey f) =
      let bs = f info
      in (uncommas $ map (parensM Parens) bs, [])

    -- | You may return tuples (up to 16-tuples) and tuples of tuples
    -- from a 'select' query.
    instance ( SqlSelect a ra
             , SqlSelect b rb
             ) => SqlSelect (a, b) (ra, rb) where
      sqlSelectCols esc (a, b) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          ]
      sqlSelectColCount = uncurry (+) . (sqlSelectColCount *** sqlSelectColCount) . fromTuple
        where
          fromTuple :: Proxy (a,b) -> (Proxy a, Proxy b)
          fromTuple = const (Proxy, Proxy)
      sqlSelectProcessRow =
        let x = getType processRow
            getType :: SqlSelect a r => (z -> Either y (r,x)) -> Proxy a
            getType = const Proxy

            colCountFst = sqlSelectColCount x

            processRow row =
                let (rowFst, rowSnd) = splitAt colCountFst row
                in (,) <$> sqlSelectProcessRow rowFst
                       <*> sqlSelectProcessRow rowSnd

        in colCountFst `seq` processRow
           -- Avoids recalculating 'colCountFst'.

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             ) => SqlSelect (a, b, c) (ra, rb, rc) where
      sqlSelectCols esc (a, b, c) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          ]
      sqlSelectColCount   = sqlSelectColCount . from3P
      sqlSelectProcessRow = fmap to3 . sqlSelectProcessRow

    from3P :: Proxy (a,b,c) -> Proxy ((a,b),c)
    from3P = const Proxy

    from3 :: (a,b,c) -> ((a,b),c)
    from3 (a,b,c) = ((a,b),c)

    to3 :: ((a,b),c) -> (a,b,c)
    to3 ((a,b),c) = (a,b,c)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             ) => SqlSelect (a, b, c, d) (ra, rb, rc, rd) where
      sqlSelectCols esc (a, b, c, d) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          ]
      sqlSelectColCount   = sqlSelectColCount . from4P
      sqlSelectProcessRow = fmap to4 . sqlSelectProcessRow

    from4P :: Proxy (a,b,c,d) -> Proxy ((a,b),(c,d))
    from4P = const Proxy

    from4 :: (a,b,c,d) -> ((a,b),(c,d))
    from4 (a,b,c,d) = ((a,b),(c,d))

    to4 :: ((a,b),(c,d)) -> (a,b,c,d)
    to4 ((a,b),(c,d)) = (a,b,c,d)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             ) => SqlSelect (a, b, c, d, e) (ra, rb, rc, rd, re) where
      sqlSelectCols esc (a, b, c, d, e) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          ]
      sqlSelectColCount   = sqlSelectColCount . from5P
      sqlSelectProcessRow = fmap to5 . sqlSelectProcessRow

    from5P :: Proxy (a,b,c,d,e) -> Proxy ((a,b),(c,d),e)
    from5P = const Proxy

    to5 :: ((a,b),(c,d),e) -> (a,b,c,d,e)
    to5 ((a,b),(c,d),e) = (a,b,c,d,e)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             ) => SqlSelect (a, b, c, d, e, f) (ra, rb, rc, rd, re, rf) where
      sqlSelectCols esc (a, b, c, d, e, f) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          ]
      sqlSelectColCount   = sqlSelectColCount . from6P
      sqlSelectProcessRow = fmap to6 . sqlSelectProcessRow

    from6P :: Proxy (a,b,c,d,e,f) -> Proxy ((a,b),(c,d),(e,f))
    from6P = const Proxy

    to6 :: ((a,b),(c,d),(e,f)) -> (a,b,c,d,e,f)
    to6 ((a,b),(c,d),(e,f)) = (a,b,c,d,e,f)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             ) => SqlSelect (a, b, c, d, e, f, g) (ra, rb, rc, rd, re, rf, rg) where
      sqlSelectCols esc (a, b, c, d, e, f, g) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          ]
      sqlSelectColCount   = sqlSelectColCount . from7P
      sqlSelectProcessRow = fmap to7 . sqlSelectProcessRow

    from7P :: Proxy (a,b,c,d,e,f,g) -> Proxy ((a,b),(c,d),(e,f),g)
    from7P = const Proxy

    to7 :: ((a,b),(c,d),(e,f),g) -> (a,b,c,d,e,f,g)
    to7 ((a,b),(c,d),(e,f),g) = (a,b,c,d,e,f,g)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             ) => SqlSelect (a, b, c, d, e, f, g, h) (ra, rb, rc, rd, re, rf, rg, rh) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          ]
      sqlSelectColCount   = sqlSelectColCount . from8P
      sqlSelectProcessRow = fmap to8 . sqlSelectProcessRow

    from8P :: Proxy (a,b,c,d,e,f,g,h) -> Proxy ((a,b),(c,d),(e,f),(g,h))
    from8P = const Proxy

    to8 :: ((a,b),(c,d),(e,f),(g,h)) -> (a,b,c,d,e,f,g,h)
    to8 ((a,b),(c,d),(e,f),(g,h)) = (a,b,c,d,e,f,g,h)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             , SqlSelect i ri
             ) => SqlSelect (a, b, c, d, e, f, g, h, i) (ra, rb, rc, rd, re, rf, rg, rh, ri) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h, i) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          , sqlSelectCols esc i
          ]
      sqlSelectColCount   = sqlSelectColCount . from9P
      sqlSelectProcessRow = fmap to9 . sqlSelectProcessRow

    from9P :: Proxy (a,b,c,d,e,f,g,h,i) -> Proxy ((a,b),(c,d),(e,f),(g,h),i)
    from9P = const Proxy

    to9 :: ((a,b),(c,d),(e,f),(g,h),i) -> (a,b,c,d,e,f,g,h,i)
    to9 ((a,b),(c,d),(e,f),(g,h),i) = (a,b,c,d,e,f,g,h,i)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             , SqlSelect i ri
             , SqlSelect j rj
             ) => SqlSelect (a, b, c, d, e, f, g, h, i, j) (ra, rb, rc, rd, re, rf, rg, rh, ri, rj) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h, i, j) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          , sqlSelectCols esc i
          , sqlSelectCols esc j
          ]
      sqlSelectColCount   = sqlSelectColCount . from10P
      sqlSelectProcessRow = fmap to10 . sqlSelectProcessRow

    from10P :: Proxy (a,b,c,d,e,f,g,h,i,j) -> Proxy ((a,b),(c,d),(e,f),(g,h),(i,j))
    from10P = const Proxy

    to10 :: ((a,b),(c,d),(e,f),(g,h),(i,j)) -> (a,b,c,d,e,f,g,h,i,j)
    to10 ((a,b),(c,d),(e,f),(g,h),(i,j)) = (a,b,c,d,e,f,g,h,i,j)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             , SqlSelect i ri
             , SqlSelect j rj
             , SqlSelect k rk
             ) => SqlSelect (a, b, c, d, e, f, g, h, i, j, k) (ra, rb, rc, rd, re, rf, rg, rh, ri, rj, rk) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h, i, j, k) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          , sqlSelectCols esc i
          , sqlSelectCols esc j
          , sqlSelectCols esc k
          ]
      sqlSelectColCount   = sqlSelectColCount . from11P
      sqlSelectProcessRow = fmap to11 . sqlSelectProcessRow

    from11P :: Proxy (a,b,c,d,e,f,g,h,i,j,k) -> Proxy ((a,b),(c,d),(e,f),(g,h),(i,j),k)
    from11P = const Proxy

    to11 :: ((a,b),(c,d),(e,f),(g,h),(i,j),k) -> (a,b,c,d,e,f,g,h,i,j,k)
    to11 ((a,b),(c,d),(e,f),(g,h),(i,j),k) = (a,b,c,d,e,f,g,h,i,j,k)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             , SqlSelect i ri
             , SqlSelect j rj
             , SqlSelect k rk
             , SqlSelect l rl
             ) => SqlSelect (a, b, c, d, e, f, g, h, i, j, k, l) (ra, rb, rc, rd, re, rf, rg, rh, ri, rj, rk, rl) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h, i, j, k, l) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          , sqlSelectCols esc i
          , sqlSelectCols esc j
          , sqlSelectCols esc k
          , sqlSelectCols esc l
          ]
      sqlSelectColCount   = sqlSelectColCount . from12P
      sqlSelectProcessRow = fmap to12 . sqlSelectProcessRow

    from12P :: Proxy (a,b,c,d,e,f,g,h,i,j,k,l) -> Proxy ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l))
    from12P = const Proxy

    to12 :: ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l)) -> (a,b,c,d,e,f,g,h,i,j,k,l)
    to12 ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l)) = (a,b,c,d,e,f,g,h,i,j,k,l)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             , SqlSelect i ri
             , SqlSelect j rj
             , SqlSelect k rk
             , SqlSelect l rl
             , SqlSelect m rm
             ) => SqlSelect (a, b, c, d, e, f, g, h, i, j, k, l, m) (ra, rb, rc, rd, re, rf, rg, rh, ri, rj, rk, rl, rm) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h, i, j, k, l, m) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          , sqlSelectCols esc i
          , sqlSelectCols esc j
          , sqlSelectCols esc k
          , sqlSelectCols esc l
          , sqlSelectCols esc m
          ]
      sqlSelectColCount   = sqlSelectColCount . from13P
      sqlSelectProcessRow = fmap to13 . sqlSelectProcessRow

    from13P :: Proxy (a,b,c,d,e,f,g,h,i,j,k,l,m) -> Proxy ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),m)
    from13P = const Proxy

    to13 :: ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),m) -> (a,b,c,d,e,f,g,h,i,j,k,l,m)
    to13 ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),m) = (a,b,c,d,e,f,g,h,i,j,k,l,m)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             , SqlSelect i ri
             , SqlSelect j rj
             , SqlSelect k rk
             , SqlSelect l rl
             , SqlSelect m rm
             , SqlSelect n rn
             ) => SqlSelect (a, b, c, d, e, f, g, h, i, j, k, l, m, n) (ra, rb, rc, rd, re, rf, rg, rh, ri, rj, rk, rl, rm, rn) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h, i, j, k, l, m, n) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          , sqlSelectCols esc i
          , sqlSelectCols esc j
          , sqlSelectCols esc k
          , sqlSelectCols esc l
          , sqlSelectCols esc m
          , sqlSelectCols esc n
          ]
      sqlSelectColCount   = sqlSelectColCount . from14P
      sqlSelectProcessRow = fmap to14 . sqlSelectProcessRow

    from14P :: Proxy (a,b,c,d,e,f,g,h,i,j,k,l,m,n) -> Proxy ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n))
    from14P = const Proxy

    to14 :: ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n)) -> (a,b,c,d,e,f,g,h,i,j,k,l,m,n)
    to14 ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n)) = (a,b,c,d,e,f,g,h,i,j,k,l,m,n)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             , SqlSelect i ri
             , SqlSelect j rj
             , SqlSelect k rk
             , SqlSelect l rl
             , SqlSelect m rm
             , SqlSelect n rn
             , SqlSelect o ro
             ) => SqlSelect (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) (ra, rb, rc, rd, re, rf, rg, rh, ri, rj, rk, rl, rm, rn, ro) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          , sqlSelectCols esc i
          , sqlSelectCols esc j
          , sqlSelectCols esc k
          , sqlSelectCols esc l
          , sqlSelectCols esc m
          , sqlSelectCols esc n
          , sqlSelectCols esc o
          ]
      sqlSelectColCount   = sqlSelectColCount . from15P
      sqlSelectProcessRow = fmap to15 . sqlSelectProcessRow

    from15P :: Proxy (a,b,c,d,e,f,g,h,i,j,k,l,m,n, o) -> Proxy ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n),o)
    from15P = const Proxy

    to15 :: ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n),o) -> (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o)
    to15 ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n),o) = (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o)

    instance ( SqlSelect a ra
             , SqlSelect b rb
             , SqlSelect c rc
             , SqlSelect d rd
             , SqlSelect e re
             , SqlSelect f rf
             , SqlSelect g rg
             , SqlSelect h rh
             , SqlSelect i ri
             , SqlSelect j rj
             , SqlSelect k rk
             , SqlSelect l rl
             , SqlSelect m rm
             , SqlSelect n rn
             , SqlSelect o ro
             , SqlSelect p rp
             ) => SqlSelect (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) (ra, rb, rc, rd, re, rf, rg, rh, ri, rj, rk, rl, rm, rn, ro, rp) where
      sqlSelectCols esc (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) =
        uncommas'
          [ sqlSelectCols esc a
          , sqlSelectCols esc b
          , sqlSelectCols esc c
          , sqlSelectCols esc d
          , sqlSelectCols esc e
          , sqlSelectCols esc f
          , sqlSelectCols esc g
          , sqlSelectCols esc h
          , sqlSelectCols esc i
          , sqlSelectCols esc j
          , sqlSelectCols esc k
          , sqlSelectCols esc l
          , sqlSelectCols esc m
          , sqlSelectCols esc n
          , sqlSelectCols esc o
          , sqlSelectCols esc p
          ]
      sqlSelectColCount   = sqlSelectColCount . from16P
      sqlSelectProcessRow = fmap to16 . sqlSelectProcessRow

    from16P :: Proxy (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p) -> Proxy ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n),(o,p))
    from16P = const Proxy

    to16 :: ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n),(o,p)) -> (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p)
    to16 ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n),(o,p)) = (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p)

    -- | Insert a 'PersistField' for every selected value.
    insertSelect :: (MonadIO m, PersistEntity a) =>
      SqlQuery (SqlExpr (Insertion a)) -> SqlPersistT m ()
    insertSelect = liftM (const ()) . rawEsqueleto INSERT_INTO . fmap EInsertFinal

    -- | Insert a 'PersistField' for every unique selected value.
    insertSelectDistinct :: (MonadIO m, PersistEntity a) =>
      SqlQuery (SqlExpr (Insertion a)) -> SqlPersistT m ()
    insertSelectDistinct = insertSelect . distinct
    {-# DEPRECATED insertSelectDistinct "Since 2.2.4: use 'insertSelect' and 'distinct'." #-}