Multiple Databases with Binds

Starting with 0.12 Flask-SQLAlchemy can easily connect to multiple databases. To achieve that it preconfigures SQLAlchemy to support multiple “binds”.

What are binds? In SQLAlchemy speak a bind is something that can execute SQL statements and is usually a connection or engine. In Flask-SQLAlchemy binds are always engines that are created for you automatically behind the scenes. Each of these engines is then associated with a short key (the bind key). This key is then used at model declaration time to assocate a model with a specific engine.

If no bind key is specified for a model the default connection is used instead (as configured by SQLALCHEMY_DATABASE_URI).

Example Configuration

The following configuration declares three database connections. The special default one as well as two others named users (for the users) and appmeta (which connects to a sqlite database for read only access to some data the application provides internally):

SQLALCHEMY_DATABASE_URI = 'postgres://localhost/main'
SQLALCHEMY_BINDS = {
    'users':        'mysqldb://localhost/users',
    'appmeta':      'sqlite:////path/to/appmeta.db'
}

Creating and Dropping Tables

The create_all() and drop_all() methods by default operate on all declared binds, including the default one. This behavior can be customized by providing the bind parameter. It takes either a single bind name, '__all__' to refer to all binds or a list of binds. The default bind (SQLALCHEMY_DATABASE_URI) is named None:

>>> db.create_all()
>>> db.create_all(bind=['users'])
>>> db.create_all(bind='appmeta')
>>> db.drop_all(bind=None)

Referring to Binds

If you declare a model you can specify the bind to use with the __bind_key__ attribute:

class User(db.Model):
    __bind_key__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True)

Internally the bind key is stored in the table’s info dictionary as 'bind_key'. This is important to know because when you want to create a table object directly you will have to put it in there:

user_favorites = db.Table('user_favorites',
    db.Column('user_id', db.Integer, db.ForeignKey('user.id')),
    db.Column('message_id', db.Integer, db.ForeignKey('message.id')),
    info={'bind_key': 'users'}
)

If you specified the __bind_key__ on your models you can use them exactly the you are used to. The model connects to the specified database connection itself.

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