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 way you are used to.  The model connects to the specified
database connection itself.
