Flask-SQLAlchemy using STI: how to automatically substitute type field

Question

I'm using Flask-SQLAlchemy==2.2 and have STI (Single Table Inheritance) configuration like this:

from flask_sqlalchemy import SQLAlchemy


db = SQLAlchemy()


class Item(db.Model):
    __tablename__ = 'item'

    id = db.Column(db.Integer, primary_key=True)
    info = db.Column(db.String(255))
    type = db.Column(db.String())

    __mapper_args__ = {
        'polymorphic_on': type
    }


class ItemA(Item):
    __mapper_args__ = {
        'polymorphic_identity': 'a'
    }


class ItemB(Item):
    __mapper_args__ = {
        'polymorphic_identity': 'b'
    }


class ItemC(Item):
    __mapper_args__ = {
        'polymorphic_identity': 'c'
    }

And I want to query items of a particular kind. I can do it like this:

Item.query.filter_by(type='b').all()

Is there any chance to do it like the following?

ItemB.query.all()

Thanks.

Answer 1

It's not working because you are accidentally using joined table inheritance instead. flask-sqlalchemy automatically sets __tablename__ for you. You need to explicitly set it to None if you want single table inheritance:

class ItemB(Item):
    __tablename__ = None
    __mapper_args__ = {
        'polymorphic_identity': 'b'
    }