Connect to multiple databases in a single schema

I'm trying to use multiple SQL databases in a single GraphQL schema. Unfortunately I haven't figured out a good way to do this.

I know I can use the `context` argument when calling the `execute` method, as documented in the [graphene docs](https://docs.graphene-python.org/en/latest/execution/execute/):

# connect to database sources
metadata = create_engine('sqlite:///metadata.sqlite3')
users = create_engine('sqlite:///users.sqlite3')
session1 = scoped_session(sessionmaker(bind=metadata))
session2 = scoped_session(sessionmaker(bind=users))

# execute query
result = schema.execute('query { allUsers { edges { node { name } } } }', context={'session': session2})

But this doesn't make sense because I'd have to determine which database to use based on the query. I should be able to pass the query in and the schema will handle that.

Flask-SQLAlchemy handles this well by binding the data source name to the SQLAlchemy model. I'd recommend doing something similar, [documentation here](http://flask-sqlalchemy.pocoo.org/2.3/binds/).

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

class User(db.Model):
__bind_key__ = 'users' # this binds the User model to the 'users' mysql database
id = db.Column(db.Integer, primary_key=True)
username = db.Column(db.String(80), unique=True)

If you don't want to do that, the `SQLAlchemyObjectType` could also be a good place to map it.

class UserNode(SQLAlchemyObjectType):
meta:
model = User
interfaces = (graphene.relay.Node,)
bind_key = 'users'

Any thoughts on working this in, or is there already a way to connect to multiple databases in a single schema?