Cross-database join in Flask-SQLAlchemy - join

Question

I'm trying to do a cross-database join in Flask-SQLAlchemy:
app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = '...Master...'
app.config['SQLALCHEMY_BINDS'] = { 'Billing': '...Billing...' }
db = SQLAlchemy(app)
class Account(db.Model):
__tablename__ = 'Accounts'
id = db.Column(db.Integer, primary_key=True)
username = db.Column(db.String(255))
class Setting(db.Model):
__tablename__ = 'Settings'
AccountId = db.Column(db.Integer, db.ForeignKey(Account.id), primary_key=True)
Enabled = db.Column(db.Boolean)
class BillingAccount(db.Model):
__tablename__ = 'Account'
__bind_key__ = 'Billing'
id = db.Column(db.Integer, primary_key=True)
AccountId = db.Column(db.Integer, db.ForeignKey(Account.id))
currency = db.Column(db.Integer)
class AccountSetting(db.Model):
__table__ = db.join(Account, AccountSetting)
id = db.column_property(Account.id, AccountSetting.AccountId)
username = Account.username
enabled = Setting.Enabled
class AccountSettingBilling(db.Model):
__table__ = db.join(Account, AccountSetting).join(BillingAccount)
id = db.column_property(Account.id, AccountSetting.AccountId, BillingAccount.AccountId)
username = Account.username
enabled = Setting.Enabled
currency = BillingAccount.currency
With this I can successfully query AccountSetting.query.all() but not AccountSettingBilling.query.all(), which fails with error 208 (MSSQL for 'object does not exist').
If I examine the generated SQL I can clearly see that it is doing a JOIN on Account.AccountId=Accounts.id when I'd expect to see some reference to Billing, e.g. Billing.Account.AccountId=Accounts.id.
Having followed Cross database join in sqlalchemy and http://pythonhosted.org/Flask-SQLAlchemy/binds.html it looks to me as though I've done things correctly. What gives?

Answer 1

You define an object db = SQLAlchemy(app) - it is Database1. You refer to it everywhere, but there is no reference to Database2. Also note code refers to columns for the join using 2 part identifiers:
Account . AccountId and Accounts . id
whereas you wish to have 3 part identifiers:
Billing . Account . AccountId and [Accounts] . Accounts . id
You are missing this property for db name from definition of each Class:
__table_args__ = {'schema': 'Accounts'}
__table_args__ = {'schema': 'Billing'}