Ecto SQLite3 v0.5.6

Ecto.Adapters.SQLite3 (Ecto SQLite3 v0.5.6) View Source

Adapter module for SQLite3.

It uses Exqlite for communicating to the database.

Options

The adapter supports a superset of the options provided by the underlying Exqlite driver.

Provided options

  • :database - The path to the database. In memory is allowed. You can use :memory or ":memory:" to designate that.
  • :journal_mode - Sets the journal mode for the sqlite connection. Can be one of the following :delete, :truncate, :persist, :memory, :wal, or :off. Defaults to :wal.
  • :temp_store - Sets the storage used for temporary tables. Default is :default. Allowed values are :default, :file, :memory.
  • :synchronous - Can be :extra, :full, :normal, or :off. Defaults to :normal.
  • :foreign_keys - Sets if foreign key checks should be enforced or not. Can be :on or :off. Default is :on.
  • :cache_size - Sets the cache size to be used for the connection. This is an odd setting as a positive value is the number of pages in memory to use and a negative value is the size in kilobytes to use. Default is -64000.
  • :cache_spill - The cache_spill pragma enables or disables the ability of the pager to spill dirty cache pages to the database file in the middle of a transaction. By default it is :on, and for most applications, it should remain so.
  • :case_sensitive_like - whether LIKE is case-sensitive or not. Can be :off or :on. Defaults to :off.
  • :auto_vacuum - Defaults to :none. Can be :none, :full or :incremental. Depending on the database size, :incremental may be beneficial.
  • :locking_mode - Defaults to :normal. Allowed values are :normal or :exclusive. See sqlite documentation for more information.
  • :secure_delete - Defaults to :off. Can be :off or :on. If :on, it will cause SQLite3 to overwrite records that were deleted with zeros.
  • :wal_auto_check_point - Sets the write-ahead log auto-checkpoint interval. Default is 1000. Setting the auto-checkpoint size to zero or a negative value turns auto-checkpointing off.
  • :busy_timeout - Sets the busy timeout in milliseconds for a connection. Default is 2000.
  • :pool_size - the size of the connection pool. Defaults to 5.

For more information about the options above, see sqlite documentation

Differences between SQLite and Ecto SQLite defaults

For the most part, the defaults we provide above match the defaults that SQLite usually ships with. However, SQLite has conservative defaults due to its need to be strictly backwards compatible, so some of them do not necessarily match "best practices". Below are the defaults we provide above that differ from the normal SQLite defaults, along with rationale.

  • :journal_mode - we use :wal, as it is vastly superior for concurrent access. SQLite usually defaults to :delete. See SQLite documentation for more info.
  • :temp_store - we use :memory, which increases performance a bit. SQLite usually defaults to :file.
  • :foreign_keys - we set it to :on, for better relational guarantees. This is also the default of the underlying Exqlite driver. SQLite usually defaults to :off for backwards compat.
  • :busy_timeout - we set it to 2000, to better enable concurrent access. This is also the default of Exqlite. SQLite usually defaults to 0.
  • :cache_size - we set it to -64000, to speed up access of data. SQLite usually defaults to -2000.

These defaults can of course be overridden, as noted above, to suit other needs.

Limitations and caveats

There are some limitations when using Ecto with SQLite that one needs to be aware of. The ones listed below are specific to Ecto usage, but it is encouraged to also view the guidance on when to use SQLite provided by the SQLite documentation, as well.

Async Sandbox testing

The Ecto SQLite3 adapter does not support async tests when used with Ecto.Adapters.SQL.Sandbox. This is due to SQLite only allowing up one write transaction at a time, which often does not work with the Sandbox approach of wrapping each test in a transaction.

LIKE match on BLOB columns

We have the DSQLITE_LIKE_DOESNT_MATCH_BLOBS compile-time option set to true, as recommended by SQLite. This means you cannot do LIKE queries on BLOB columns.

Case sensitivity

Case sensitivty for LIKE is off by default, and controlled by the :case_sensitive_like option outlined above.

However, for equality comparison, case sensitivity is always on. If you want to make a column not be case sensitive, for email storage for example, you can make it case insensitive by using the COLLATE NOCASE option in SQLite. This is configured via the :collate option.

So instead of:

add :email, :string

You would do:

add :email, :string, collate: :nocase

Check constraints

SQLite3 supports specifying check constraints on the table or on the column definition. We currently only support adding a check constraint via a column definition, since the table definition approach only works at table-creation time and cannot be added at table-alter time. You can see more information in the SQLite3 CREATE TABLE documentation.

Because of this, you cannot add a constraint via the normal Ecto.Migration.constraint/3 method, as that operates via ALTER TABLE ADD CONSTRAINT, and this type of ALTER TABLE operation SQLite3 does not support. You can however get the full functionality by adding a constraint at the column level, specifying the name and expression. Per the SQLite3 documentation, there is no functional difference between a column or table constraint.

Thus, adding a check constraint for a new column is as simple as:

add :email, :string, check: %{name: "test_constraint", expr: "email != 'test@example.com')"}

Handling foreign key constraints in changesets

Unfortunately, unlike other databases, SQLite3 does not provide the precise name of the constraint violated, but only the columns within that constraint (if it provides any information at all). Because of this, changeset functions like Ecto.Changeset.foreign_key_constraint/3 may not work at all.

This is because the above functions depend on the Ecto Adapter returning the name of the violated constraint, which you annotate in your changeset so that Ecto can convert the constraint violation into the correct updated changeset when the constraint is hit during a Ecto.Repo.update/2 or Ecto.Repo.insert/2 operation. Since we cannot get the name of the violated constraint back from SQLite3 at INSERT or UPDATE time, there is no way to effectively use these changeset functions. This is a SQLite3 limitation.

See this GitHub issue for more details.

Schemaless queries

Using schemaless Ecto queries will not work well with SQLite. This is because the Ecto SQLite adapter relies heavily on the schema to support a rich array of Elixir types, despite the fact SQLite only has five storage classes. The query will still work and return data, but you will need to do this mapping on your own.