AbstractConcreteBase and query_expression()

[Benjamin Beguin]

Hi there,

I'm facing an issue with the use of AbstractConcreteBase class and a @declarred_attr returning a query_expression()

class A(AbstractConcreteBase, Base):
    @declared_attr
    def foo(cls):
        return query_expression()


class B(A):
 ...


class C(A):
 ...


objects = db.session.query(A).options(with_expression(A.foo, my_expression)).all()

B and C objects returned have the property foo but it is None

The generated SQL query returns a value for the expression but the mapping seems to be not effective 

Am I doing something wrong ?

[Mike Bayer]

I don't really know, because AbstractConcreteBase works pretty poorly and it's better to use other patterns.

Try this workaround, which I have no idea if it helps in this case:

class A(...):

    # ...

    @classmethod

    def __declare_last__(cls):

        cls.__mapper__.with_polymorphic = ("*", cls.__mapper__.local_table)

otherwise provide a more complete example and I'll try to diagnose why it doesnt work for this case.

--

SQLAlchemy -

The Python SQL Toolkit and Object Relational Mapper

 

http://www.sqlalchemy.org/

 

To post example code, please provide an MCVE: Minimal, Complete, and Verifiable Example. See http://stackoverflow.com/help/mcve for a full description.

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[Mike Bayer]

I'm not able to reproduce the issue you describe.  Below is an MCVE, can you please modify it to show the failure you are getting?   The .foo attribute on B is returned:

from sqlalchemy import Column

from sqlalchemy import create_engine

from sqlalchemy import Integer

from sqlalchemy import String

from sqlalchemy.ext.declarative import AbstractConcreteBase

from sqlalchemy.ext.declarative import declarative_base

from sqlalchemy.ext.declarative import declared_attr

from sqlalchemy.orm import configure_mappers

from sqlalchemy.orm import query_expression

from sqlalchemy.orm import Session

from sqlalchemy.orm import with_expression

Base = declarative_base()

class A(AbstractConcreteBase, Base):

    @declared_attr

    def foo(cls):

        return query_expression()

class B(A):

    __tablename__ = "b"

    id = Column(Integer, primary_key=True)

    data = Column(String(50))

    __mapper_args__ = {"polymorphic_identity": "b"}

class C(A):

    __tablename__ = "c"

    id = Column(Integer, primary_key=True)

    __mapper_args__ = {"polymorphic_identity": "c"}

configure_mappers()

e = create_engine("sqlite://", echo=True)

Base.metadata.create_all(e)

session = Session(e)

session.add(B(id=1, data='b data'))

session.commit()

b1 = session.query(A).options(with_expression(A.foo, B.data + ' hi')).all()[0]

assert b1.foo == "b data hi"

On Wed, Mar 25, 2020, at 12:30 PM, Benjamin Beguin wrote: