# SpatiaLite Tutorial

GeoAlchemy 2’s main target is PostGIS. But GeoAlchemy 2 also supports SpatiaLite, the spatial extension to SQLite. This tutorial describes how to use GeoAlchemy 2 with SpatiaLite. It’s based on the ORM Tutorial, which you may want to read first.

## Connect to the DB

Just like when using PostGIS connecting to a SpatiaLite database requires an `Engine`. An engine for the SpatiaLite dialect can be created in two ways. Using the plugin provided by `GeoAlchemy2` (see Plugin for more details):

    >>> from sqlalchemy import create_engine
    >>> engine = create_engine(
    ...     "sqlite:///gis.db",
    ...     echo=True,
    ...     plugins=["geoalchemy2"]
    ... )

Or by attaching the listeners manually:

    >>> from geoalchemy2 import load_spatialite
    >>> from sqlalchemy import create_engine
    >>> from sqlalchemy.event import listen
    >>>
    >>> engine = create_engine("sqlite:///gis.db", echo=True)
    >>> listen(engine, "connect", load_spatialite)

The call to `create_engine` creates an engine bound to the database file `gis.db`. After that a `connect` listener is registered on the engine. The listener is responsible for loading the SpatiaLite extension, which is a necessary operation for using SpatiaLite through SQL. The path to the `mod_spatialite` file should be stored in the `SPATIALITE_LIBRARY_PATH` environment variable before using the `load_spatialite` function.

At this point you can test that you are able to connect to the database:

    >> conn = engine.connect()

Note that this call will internally call the `load_spatialite` function, which can take some time to execute on a new database because it actually calls the `InitSpatialMetaData` function from SpatiaLite (it is possible to reduce this time by loading only the required SRIDs, see `geoalchemy2.admin.dialects.sqlite.load_spatialite()`). Then you can also check that the `gis.db` SQLite database file was created on the file system.

Note that when `InitSpatialMetaData` is executed again it will report an error:

    InitSpatiaMetaData() error:"table spatial_ref_sys already exists"

You can safely ignore that error.

Before going further we can close the current connection:

    >>> conn.close()

## Declare a Mapping

Now that we have a working connection we can go ahead and create a mapping between a Python class and a database table:

    >>> from sqlalchemy.orm import declarative_base
    >>> from sqlalchemy import Column, Integer, String
    >>> from geoalchemy2 import Geometry
    >>>
    >>> Base = declarative_base()
    >>>
    >>> class Lake(Base):
    ...     __tablename__ = "lake"
    ...     id = Column(Integer, primary_key=True)
    ...     name = Column(String)
    ...     geom = Column(Geometry(geometry_type="POLYGON"))

From the user point of view this works in the same way as with PostGIS. The difference is that internally the `RecoverGeometryColumn` and `DiscardGeometryColumn` management functions will be used for the creation and removal of the geometry column.

## Function mapping

Several functions have different names in SpatiaLite than in PostGIS. The GeoAlchemy 2 package is based on the PostGIS syntax but it is possible to automatically translate the queries into SpatiaLite ones. For example, the function `ST_GeomFromEWKT` is automatically translated into `GeomFromEWKT`. Unfortunately, only a few functions are automatically mapped (mainly the ones internally used by GeoAlchemy 2). Nevertheless, it is possible to define new mappings in order to translate the queries automatically. Here is an example to register a mapping for the `ST_Buffer` function:

    >>> geoalchemy2.functions.register_sqlite_mapping(
    ...     {"ST_Buffer": "Buffer"}
    ... )

After this command, all `ST_Buffer` calls in the queries will be translated to `Buffer` calls when the query is executed on a SQLite DB.

A more complex example is provided for when the PostGIS function should be mapped depending on the given parameters. For example, the `ST_Buffer` function can actually be translate into either the `Buffer` function or the `SingleSidedBuffer` function (only when `side=right` or `side=left` is passed). See the Function translation for specific dialect example in the gallery.

## GeoPackage format

Starting from the version `4.2` of Spatialite, it is possible to use GeoPackage files as DB containers. GeoAlchemy 2 is able to handle most of the GeoPackage features automatically if the GeoPackage dialect is used (i.e. the DB URL starts with `gpkg:///`) and the SpatiaLite extension is loaded. Usually, this extension should be loaded using the the `GeoAlchemy2` plugin (see connect section) or by attaching the `load_spatialite_gpkg` listener to the engine:

    >>> from geoalchemy2 import load_spatialite_gpkg
    >>> from sqlalchemy import create_engine
    >>> from sqlalchemy.event import listen
    >>>
    >>> engine = create_engine("gpkg:///gis.gpkg", echo=True)
    >>> listen(engine, "connect", load_spatialite_gpkg)

When using the `load_spatialite_gpkg` listener on a DB recognized as a GeoPackage, specific processes are activated:

- the base tables are created if they are missing,

- the `Amphibious` mode is enabled using the `EnableGpkgAmphibiousMode` function,

- the `VirtualGPKG` wrapper is activated using the `AutoGpkgStart` function.

After that it should be possible to use a GeoPackage the same way as a standard SpatiaLite database. GeoAlchemy 2 should be able to handle the following features in a transparent way for the user:

- create/drop spatial tables,

- automatically create/drop spatial indexes if required,

- reflect spatial tables,

- use spatial functions on inserted geometries.

Note

If you want to use the `ST_Transform` function you should call the `geoalchemy2.admin.dialects.geopackage.create_spatial_ref_sys_view()` first.

## Further Reference

- GeoAlchemy 2 ORM Tutotial: ORM Tutorial

- GeoAlchemy 2 Spatial Functions Reference: Spatial Functions

- GeoAlchemy 2 Spatial Operators Reference: Spatial Operators

- GeoAlchemy 2 Elements Reference: Elements

- SpatiaLite 4.3.0 SQL functions reference list