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# Library Database API

This page describes the internal API of beets’ core database features. It doesn’t exhaustively document the API, but is aimed at giving an overview of the architecture to orient anyone who wants to dive into the code.

The `Library` object is the central repository for data in beets. It represents a database containing songs, which are `Item` instances, and groups of items, which are `Album` instances.

## The Library Class

The `Library` is typically instantiated as a singleton. A single invocation of beets usually has only one `Library`. It’s powered by `dbcore.Database` under the hood, which handles the SQLite abstraction, something like a very minimal ORM. The library is also responsible for handling queries to retrieve stored objects.

### Overview

You can add new items or albums to the library via the `Library.add()` and `Library.add_album()` methods.

You may also query the library for items and albums using the `Library.items()`, `Library.albums()`, `Library.get_item()` and `Library.get_album()` methods.

Any modifications to the library must go through a `Transaction` object, which you can get using the `Library.transaction()` context manager.

## Model Classes

The two model entities in beets libraries, `Item` and `Album`, share a base class, `LibModel`, that provides common functionality. That class itself specialises `beets.dbcore.Model` which provides an ORM-like abstraction.

To get or change the metadata of a model (an item or album), either access its attributes (e.g., `print(album.year)` or `album.year = 2012`) or use the `dict`-like interface (e.g. `item['artist']`).

### Model base

Models use dirty-flags to track when the object’s metadata goes out of sync with the database. The dirty dictionary maps field names to booleans indicating whether the field has been written since the object was last synchronized (via load or store) with the database. This logic is implemented in the model base class `LibModel` and is inherited by both `Item` and `Album`.

We provide CRUD-like methods for interacting with the database:

- `LibModel.store()`

- `LibModel.load()`

- `LibModel.remove()`

- `LibModel.add()`

The base class `beets.dbcore.Model` has a `dict`-like interface, so normal the normal mapping API is supported:

- `LibModel.keys()`

- `LibModel.update()`

- `LibModel.items()`

- `LibModel.get()`

### Item

Each `Item` object represents a song or track. (We use the more generic term item because, one day, beets might support non-music media.) An item can either be purely abstract, in which case it’s just a bag of metadata fields, or it can have an associated file (indicated by `item.path`).

In terms of the underlying SQLite database, items are backed by a single table called items with one column per metadata fields. The metadata fields currently in use are listed in `library.py` in `Item._fields`.

To read and write a file’s tags, we use the MediaFile library. To make changes to either the database or the tags on a file, you update an item’s fields (e.g., `item.title = "Let It Be"`) and then call `item.write()`.

Items also track their modification times (mtimes) to help detect when they become out of sync with on-disk metadata, mainly to speed up the update (which needs to check whether the database is in sync with the filesystem). This feature turns out to be sort of complicated.

For any `Item`, there are two mtimes: the on-disk mtime (maintained by the OS) and the database mtime (maintained by beets). Correspondingly, there is on-disk metadata (ID3 tags, for example) and DB metadata. The goal with the mtime is to ensure that the on-disk and DB mtimes match when the on-disk and DB metadata are in sync; this lets beets do a quick mtime check and avoid rereading files in some circumstances.

Specifically, beets attempts to maintain the following invariant:

> If the on-disk metadata differs from the DB metadata, then the on-disk mtime must be greater than the DB mtime.

As a result, it is always valid for the DB mtime to be zero (assuming that real disk mtimes are always positive). However, whenever possible, beets tries to set `db_mtime = disk_mtime` at points where it knows the metadata is synchronized. When it is possible that the metadata is out of sync, beets can then just set `db_mtime = 0` to return to a consistent state.

This leads to the following implementation policy:

> - On every write of disk metadata (`Item.write()`), the DB mtime is updated to match the post-write disk mtime.

> - Same for metadata reads (`Item.read()`).

> - On every modification to DB metadata (`item.field = ...`), the DB mtime is reset to zero.

### Album

An `Album` is a collection of Items in the database. Every item in the database has either zero or one associated albums (accessible via `item.album_id`). An item that has no associated album is called a singleton. Changing fields on an album (e.g. `album.year = 2012`) updates the album itself and also changes the same field in all associated items.

An `Album` object keeps track of album-level metadata, which is (mostly) a subset of the track-level metadata. The album-level metadata fields are listed in `Album._fields`. For those fields that are both item-level and album-level (e.g., `year` or `albumartist`), every item in an album should share the same value. Albums use an SQLite table called `albums`, in which each column is an album metadata field.

Note

The `Album.items()` method is not inherited from `LibModel.items()` for historical reasons.

### Transactions

The `Library` class provides the basic methods necessary to access and manipulate its contents. To perform more complicated operations atomically, or to interact directly with the underlying SQLite database, you must use a transaction (see this blog post for motivation). For example

    lib = Library()
    with lib.transaction() as tx:
        items = lib.items(query)
        lib.add_album(list(items))

The `Transaction` class is a context manager that provides a transactional interface to the underlying SQLite database. It is responsible for managing the transaction’s lifecycle, including beginning, committing, and rolling back the transaction if an error occurs.

## Queries

To access albums and items in a library, we use Queries. In beets, the `Query` abstract base class represents a criterion that matches items or albums in the database. Every subclass of `Query` must implement two methods, which implement two different ways of identifying matching items/albums.

The `clause()` method should return an SQLite `WHERE` clause that matches appropriate albums/items. This allows for efficient batch queries. Correspondingly, the `match(item)` method should take an `Item` object and return a boolean, indicating whether or not a specific item matches the criterion. This alternate implementation allows clients to determine whether items that have already been fetched from the database match the query.

There are many different types of queries. Just as an example, `FieldQuery` determines whether a certain field matches a certain value (an equality query). `AndQuery` (like its abstract superclass, `CollectionQuery`) takes a set of other query objects and bundles them together, matching only albums/items that match all constituent queries.

Beets has a human-writable plain-text query syntax that can be parsed into `Query` objects. Calling `AndQuery.from_strings` parses a list of query parts into a query object that can then be used with `Library` objects.