# `Ecto.Adapters.SQLite3`
[🔗](https://github.com/elixir-sqlite/ecto_sqlite3/blob/v0.23.0/lib/ecto/adapters/sqlite3.ex#L1)

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.
  * `:default_transaction_mode` - one of `:deferred` (default), `:immediate`, or
    `:exclusive`. If a mode is not specified in a call to `Repo.transaction/2`, this
    will be the default transaction mode.
  * `: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][1] 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`.
  * `:binary_id_type` - Defaults to `:string`. Determines how binary IDs are stored in
    the database and the type of `:binary_id` columns. See the
    [section on binary ID types](#module-binary-id-types) for more details.
  * `:uuid_type` - Defaults to `:string`. Determines the type of `:uuid` columns.
    Possible values and column types are the same as for
    [binary IDs](#module-binary-id-types).
  * `:map_type` - Defaults to `:string`. Determines the type of `:map` columns.
    Set to `:binary` to use the [JSONB](https://sqlite.org/jsonb.html)
    storage format.
  * `:array_type` - Defaults to `:string`. Determines the type of `:array` columns.
    Arrays are serialized using JSON. Set to `:binary` to use the
    [JSONB](https://sqlite.org/jsonb.html) storage format.
  * `:datetime_type` - Defaults to `:iso8601`. Determines how datetime fields are
    stored in the database. The allowed values are `:iso8601` and `:text_datetime`.
    `:iso8601` corresponds to a string of the form `YYYY-MM-DDThh:mm:ss` and
    `:text_datetime` corresponds to a string of the form `YYYY-MM-DD hh:mm:ss`
  * `:load_extensions` - list of paths identifying extensions to load. Defaults to `[]`.
     The provided list will be merged with the global extensions list, set on
     `:exqlite, :load_extensions`. Be aware that the path should handle pointing to a
     library compiled for the current architecture. See `Exqlite.Connection.connect/1`
     for more.

For more information about the options above, see [sqlite documentation][1]

### 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][2] 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.

[1]: https://www.sqlite.org/pragma.html
[2]: https://sqlite.org/wal.html

### Binary ID types

The `:binary_id_type` configuration option allows configuring how `:binary_id` fields
are stored in the database as well as the type of the column in which these IDs will
be stored. The possible values are:

* `:string` - IDs are stored as strings, and the type of the column is `TEXT`. This is
  the default.
* `:binary` - IDs are stored in their raw binary form, and the type of the column is `BLOB`.

The main differences between the two formats are as follows:
* When stored as binary, UUIDs require much less space in the database. IDs stored as
  strings require 36 bytes each, while IDs stored as binary only require 16 bytes.
* Because SQLite does not have a dedicated UUID type, most clients cannot represent
  UUIDs stored as binary in a human readable format. Therefore, IDs stored as strings
  may be easier to work with if manual manipulation is required.

## 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][4] provided
by the SQLite documentation, as well.

### In memory robustness

When using the Ecto SQLite3 adapter with the database set to `:memory` it
is possible that a crash in a process performing a query in the Repo will
cause the database to be destroyed. This makes the `:memory` function
unsuitable when it is expected to survive potential process crashes (for
example a crash in a Phoenix request)

### 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 `SQLITE_LIKE_DOESNT_MATCH_BLOBS` compile-time definition option set to true,
as [recommended by SQLite][3]. This means you cannot do `LIKE` queries on `BLOB` columns.

### Case sensitivity

Case sensitivity 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`][6] 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](https://sqlite.org/lang_createtable.html).

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 `c:Ecto.Repo.update/2` or `c: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](https://github.com/elixir-sqlite/ecto_sqlite3/issues/42) for
more details.

### Schemaless queries

Using [schemaless Ecto queries][7] 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][5]. The query will still
work and return data, but you will need to do this mapping on your own.

### Transaction mode

By default, [SQLite transactions][8] run in `DEFERRED` mode. However, in 
web applications with a balanced load of reads and writes, using  `IMMEDIATE` 
mode may yield better performance.

Here are several ways to specify a different transaction mode:

**Pass `mode: :immediate` to `Repo.transaction/2`:** Use this approach to set 
the transaction mode for individual transactions.

    Multi.new()
    |> Multi.run(:example, fn _repo, _changes_so_far ->
      # ... do some work ...
    end)
    |> Repo.transaction(mode: :immediate)

**Define custom transaction functions:** Create wrappers, such as 
`Repo.immediate_transaction/2` or `Repo.deferred_transaction/2`, to easily 
apply different modes where needed.

    defmodule MyApp.Repo do
      def immediate_transaction(fun_or_multi) do
        transaction(fun_or_multi, mode: :immediate)
      end

      def deferred_transaction(fun_or_multi) do
        transaction(fun_or_multi, mode: :deferred)
      end
    end

**Set a global default:** Configure `:default_transaction_mode` to apply a 
preferred mode for all transactions, unless explicitly passed a different
`:mode` to `Repo.transaction/2`.

    config :my_app, MyApp.Repo,
      database: "path/to/my/database.db",
      default_transaction_mode: :immediate

[3]: https://www.sqlite.org/compile.html
[4]: https://www.sqlite.org/whentouse.html
[5]: https://www.sqlite.org/datatype3.html
[6]: https://www.sqlite.org/datatype3.html#collating_sequences
[7]: https://hexdocs.pm/ecto/schemaless-queries.html
[8]: https://www.sqlite.org/lang_transaction.html#deferred_immediate_and_exclusive_transactions

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*Consult [api-reference.md](api-reference.md) for complete listing*