[sqlalchemy] Polymorphic AbstractConcreteBase , eager load , conditions , etc

[Harshvardhan Gupta]

I tried to use AbstractConcreteBase for polymorphic relationships , but I 
am getting errors. The examples in sqlalchemy cover normal polymorphism 
well, but not those with Abstract Base classes. 


I have already asked a question on stack overflow  
<https://stackoverflow.com/questions/49141410/correctly-eager-loading-polymorphic-relationships>
. 

The gist of the question is: 

class OrderItem(Dictifiable, AbstractConcreteBase, db.Model):
    pass


class OrderTestItem(OrderItem):
    order_id = Column(Integer, ForeignKey("order.id"), primary_key=True)
    test_id = Column(Integer, ForeignKey("test.id"), primary_key=True)

    test = relationship('Test')
    order = relationship('Order')

    __mapper_args__ = {
        'polymorphic_identity': 'order_test_item',
        'concrete': True
    }



class Order(Dictifiable, db.Model): # This class has a relation to the 
polymorphic class


    id = Column(Integer, Sequence('user_id_seq'), primary_key=True)

    user_id = Column(Integer, ForeignKey('user.id'))

    user = relationship('User')
    items = relationship('OrderItem')


I query like : 

pjoin = polymorphic_union({
    'order_test_item': OrderTestItem.__table__,
}, 'type', 'pjoin')

order_items = 
with_polymorphic(OrderItem,[OrderTestItem],selectable=pjoin,flat=True)

And my actual query : 

order = Order.query.join(Order.items.of_type(order_items)).all()


I would like to know what the correct way to query these tables is, how to 
eager load polymorphic tables, and how to filter on the relationships. 

I plan to send a pull request with an example of these test cases after I know 
the answers myself. 


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