High-load SQLAlchemy subtypes collection

Question

I have this (simplified) user / group model in one of my projects:

import sqlalchemy
import sqlalchemy.ext.declarative
import sqlalchemy.orm
import sqlalchemy.schema

Base = sqlalchemy.ext.declarative.declarative_base()


class Party(Base):
    __tablename__ = 'Party'
    type = sqlalchemy.Column(
        sqlalchemy.String(1),
        nullable=False,
    )
    name = sqlalchemy.Column(
        sqlalchemy.String(255),
        primary_key=True,
    )

    __mapper_args__ = {
        'polymorphic_identity': 'S',
        'polymorphic_on': type,
    }


class User(Party):
    __tablename__ = 'User'
    name = sqlalchemy.Column(
        sqlalchemy.String(255),
        sqlalchemy.schema.ForeignKey(Party.name),
        primary_key=True,
    )

    __mapper_args__ = {
        'polymorphic_identity': 'U',
    }


class Group(Party):
    __tablename__ = 'User'
    name = sqlalchemy.Column(
        sqlalchemy.String(255),
        sqlalchemy.schema.ForeignKey(Party.name),
        primary_key=True,
    )

    __mapper_args__ = {
        'polymorphic_identity': 'G',
    }


class UserGroup(Base):
    __tablename__ = 'UserGroup'
    user_name = sqlalchemy.Column(
        sqlalchemy.String(255),
        sqlalchemy.schema.ForeignKey(User.name),
        primary_key=True,
    )
    group_name = sqlalchemy.Column(
        sqlalchemy.String(255),
        sqlalchemy.schema.ForeignKey(Group.name),
        primary_key=True,
    )

    user = sqlalchemy.orm.relationship(User, backref='user_groups')

I am encountering a problem when trying to load a property user_groups Userin a polymorphic request:

party_poly = sqlalchemy.orm.with_polymorphic(Party, [User, Group])
results = session.query(party_poly) \
    .options(sqlalchemy.orm.subqueryload(User.user_groups)) \
    .all()

Indicated error:

Cannot find property 'user_groups' for any object specified in this query. Note that the full path from root (Mapper | Party | Party) to the target must be specified.

Through the trial version and error, I received this request to work by changing

party_poly = sqlalchemy.orm.with_polymorphic(Party, [User, Group])

to

party_poly = sqlalchemy.orm.with_polymorphic(Party, [User, Group],
                                             _use_mapper_path=True)

So the question is: can one rely on an undocumented argument argument _use_mapper_path? Does anyone know what he is doing?

Is there a better way to do what I'm trying to do? If there is, I would love to hear it!

Answer 1

_use_mapper_path AliasedClass . AliasedClass (, , backref user_groups).

backrefs . :

class A (db.Model):

    __tablename__ = 'a'

    something = db.Column( db.BigInteger( unsigned=True ), primary_key=True )

    b         = db.relationship( 'B', use_list=False )

class B (db.Model):

    __tablename__ = 'b'

    a_id      = db.Column( db.BigInteger( unsigned=True ), db.ForeignKey( 'a.id' ), primary_key=True )

    a         = db.relationship( 'A' )

, , .