Explicit Master-Master DB setup with Flask and SQLAlchemy, hopefully with Flask-SQLAlchemy

The code below is what I ended up with to have this functionality.

A few notes:

• I changed get_table_for_bind to bind all tables without an explicit __bind_key__ to all the binds. This is done in order to be able to call db.create_all() or db.drop_all() and create/drop the tables in all the DBs. In order for this to work and not break the default DB selection, when not specifying a specific bind, get_binds was changed to map the None bind again after the original implementation, to override the Table->Bind mapping.
• If you don't specify a using_bind everything should work with the default DB.
• SQLAlchemy mapped objects keep a reference to the session and state so you can't really add the same object to two DBs. I made a copy of the object before adding it in order to persist it in two DBs. Not sure if there is some better way to do this.
• I haven't fully tested this and this might break some other functionality I'm not using or not aware of.

flask-sqlalchemy overrides:

from flask_sqlalchemy import SQLAlchemy, SignallingSession, get_statefrom flask_sqlalchemy._compat import itervaluesclass UsingBindSignallingSession(SignallingSession):    def get_bind(self, mapper=None, clause=None):        if self._name:            _eng = get_state(self.app).db.get_engine(self.app,bind=self._name)            return _eng        else:            return super(UsingBindSignallingSession, self).get_bind(mapper, clause)    _name = None    def using_bind(self, name):        self._name = name        return selfclass UsingBindSQLAlchemy(SQLAlchemy):    def create_session(self, options):        return UsingBindSignallingSession(self, **options)    def get_binds(self, app=None):        retval = super(UsingBindSQLAlchemy, self).get_binds(app)        # get the binds for None again in order to make sure that it is the default bind for tables        # without an explicit bind        bind = None        engine = self.get_engine(app, bind)        tables = self.get_tables_for_bind(bind)        retval.update(dict((table, engine) for table in tables))        return retval    def get_tables_for_bind(self, bind=None):        """Returns a list of all tables relevant for a bind.        Tables without an explicit __bind_key__ will be bound to all binds.        """        result = []        for table in itervalues(self.Model.metadata.tables):            # if we don't have an explicit __bind_key__ bind this table to all databases            if table.info.get('bind_key') == bind or table.info.get('bind_key') == None:                result.append(table)        return resultdb = UsingBindSQLAlchemy()

Now you can do this:

# This is the default DBSQLALCHEMY_DATABASE_URI=YOUR_MAIN_DB_URI_CONNECT_STRING# Master1 and Master2SQLALCHEMY_BINDS = { 'master1':YOUR_MASTER1_DB_URI_CONNECT_STRING, 'master2':YOUR_MASTER2_DB_URI_CONNECT_STRING }# Tables without __bind_key__ will be dropped/created on all DBs (default, master1, master2)db.drop_all()db.create_all()s = db.session().using_bind('master1')s.add(SOME_OBJECT)s.commit()s = db.session().using_bind('master2')s.add(SOME_OBJECT_CLONE) # a clone of the original object, before the first add()s.commit()# and the default DB, as alwaysdb.session.add(SOME_OTHER_OBJECT)db.session.commit()