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Configuration
=============
Configuration Keys
------------------
Configuration is loaded from the Flask ``app.config`` when :meth:`.SQLAlchemy.init_app`
is called. The configuration is not read again after that. Therefore, all configuration
must happen before initializing the application.
.. module:: flask_sqlalchemy.config
.. data:: SQLALCHEMY_DATABASE_URI
The database connection URI used for the default engine. It can be either a string
or a SQLAlchemy ``URL`` instance. See below and :external:doc:`core/engines` for
examples.
At least one of this and :data:`SQLALCHEMY_BINDS` must be set.
.. versionchanged:: 3.0
No longer defaults to an in-memory SQLite database if not set.
.. data:: SQLALCHEMY_ENGINE_OPTIONS
A dict of arguments to pass to :func:`sqlalchemy.create_engine` for the default
engine.
This takes precedence over the ``engine_options`` argument to :class:`.SQLAlchemy`,
which can be used to set default options for all engines.
.. versionchanged:: 3.0
Only applies to the default bind.
.. versionadded:: 2.4
.. data:: SQLALCHEMY_BINDS
A dict mapping bind keys to engine options. The value can be a string or a
SQLAlchemy ``URL`` instance. Or it can be a dict of arguments, including the ``url``
key, that will be passed to :func:`sqlalchemy.create_engine`. The ``None`` key can
be used to configure the default bind, but :data:`SQLALCHEMY_ENGINE_OPTIONS` and
:data:`SQLALCHEMY_DATABASE_URI` take precedence.
At least one of this and :data:`SQLALCHEMY_DATABASE_URI` must be set.
.. versionadded:: 0.12
.. data:: SQLALCHEMY_ECHO
The default value for ``echo`` and ``echo_pool`` for every engine. This is useful to
quickly debug the connections and queries issued from SQLAlchemy.
.. versionchanged:: 3.0
Sets ``echo_pool`` in addition to ``echo``.
.. data:: SQLALCHEMY_RECORD_QUERIES
If enabled, information about each query during a request will be recorded. Use
:func:`.get_recorded_queries` to get a list of queries that were issued during the
request.
.. versionchanged:: 3.0
Not enabled automatically in debug or testing mode.
.. data:: SQLALCHEMY_TRACK_MODIFICATIONS
If enabled, all ``insert``, ``update``, and ``delete`` operations on models are
recorded, then sent in :data:`.models_committed` and
:data:`.before_models_committed` signals when ``session.commit()`` is called.
This adds a significant amount of overhead to every session. Prefer using
SQLAlchemy's :external:doc:`orm/events` directly for the exact information you need.
.. versionchanged:: 3.0
Disabled by default.
.. versionadded:: 2.0
.. versionchanged:: 3.1
Removed ``SQLALCHEMY_COMMIT_ON_TEARDOWN``.
.. versionchanged:: 3.0
Removed ``SQLALCHEMY_NATIVE_UNICODE``, ``SQLALCHEMY_POOL_SIZE``,
``SQLALCHEMY_POOL_TIMEOUT``, ``SQLALCHEMY_POOL_RECYCLE``, and
``SQLALCHEMY_MAX_OVERFLOW``.
Connection URL Format
---------------------
See SQLAlchemy's documentation on :external:doc:`core/engines` for a complete
description of syntax, dialects, and options.
A basic database connection URL uses the following format. Username, password, host, and
port are optional depending on the database type and configuration.
.. code-block:: text
dialect://username:password@host:port/database
Here are some example connection strings:
.. code-block:: text
# SQLite, relative to Flask instance path
sqlite:///project.db
# PostgreSQL
postgresql://scott:tiger@localhost/project
# MySQL / MariaDB
mysql://scott:tiger@localhost/project
SQLite does not use a user or host, so its URLs always start with _three_ slashes
instead of two. The ``dbname`` value is a file path. Absolute paths start with a
_fourth_ slash (on Linux or Mac). Relative paths are relative to the Flask application's
:attr:`~flask.Flask.instance_path`.
Default Driver Options
----------------------
Some default options are set for SQLite and MySQL engines to make them more usable by
default in web applications.
SQLite relative file paths are relative to the Flask instance path instead of the
current working directory. In-memory databases use a static pool and
``check_same_thread`` to work across requests.
MySQL (and MariaDB) servers are configured to drop connections that have been idle for
8 hours, which can result in an error like ``2013: Lost connection to MySQL server
during query``. A default ``pool_recycle`` value of 2 hours (7200 seconds) is used to
recreate connections before that timeout.
Engine Configuration Precedence
-------------------------------
Because Flask-SQLAlchemy has support for multiple engines, there are rules for which
config overrides other config. Most applications will only have a single database and
only need to use :data:`SQLALCHEMY_DATABASE_URI` and :data:`SQLALCHEMY_ENGINE_OPTIONS`.
- If the ``engine_options`` argument is given to :class:`.SQLAlchemy`, it sets default
options for *all* engines. :data:`SQLALCHEMY_ECHO` sets the default value for both
``echo`` and ``echo_pool`` for all engines.
- The options for each engine in :data:`.SQLALCHEMY_BINDS` override those defaults.
- :data:`.SQLALCHEMY_ENGINE_OPTIONS` overrides the ``None`` key in
``SQLALCHEMY_BINDS``, and :data:`.SQLALCHEMY_DATABASE_URI` overrides the ``url`` key
in that engine's options.
Timeouts
--------
Certain databases may be configured to close inactive connections after a period of
time. MySQL and MariaDB are configured for this by default, but database services may
also configure this type of limit. This can result in an error like
``2013: Lost connection to MySQL server during query``.
If you encounter this error, try setting ``pool_recycle`` in the engine options to
a value less than the database's timeout.
Alternatively, you can try setting ``pool_pre_ping`` if you expect the database to close
connections often, such as if it's running in a container that may restart.
See SQAlchemy's docs on `dealing with disconnects`_ for more information.
.. _dealing with disconnects: https://docs.sqlalchemy.org/core/pooling.html#dealing-with-disconnects
|
