Multiple mappers found by get_mapper - model inheritance

[Piotr]

I'm updating "sqlalchemy_django_query" for SQLAlchemy 1.4 and aside of that I found a peculiar problem - when I call get_mapper on  a table (AnnotatedTable) I get a ValueError due to multiple mappers found.

The model is inherited by two other models and get_mapper finds mappers for all 3 models due to this:

class Conversation(db.Base):
    __tablename__ = 'conversation'
    id = sqlalchemy.Column(postgres_dialect.UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
    ...
    type = sqlalchemy.Column(sqlalchemy.Text, nullable=False)

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


...


class ContactFormConversation(Conversation):
    __tablename__ = 'contact_form_conversation'
    id = sqlalchemy.Column(sqlalchemy.ForeignKey('conversation.id'), primary_key=True, nullable=False)
    ...

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


...


class RatingConversation(Conversation):
    __tablename__ = 'rating_conversation'
    id = sqlalchemy.Column(sqlalchemy.ForeignKey('conversation.id'), primary_key=True, nullable=False)
    ...

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

Should the model structure be done differently?

[Mike Bayer]

I would need to see what get_mapper() is doing.   the best way to associate the relevant table to a mapper is to look at inspect(Class).mapper.local_table.

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[piotr maliński]

AnnotatedTable doesn't have a mapper so I call:

get_mapper(annotated_table_object_here) To then map a column name to a column on a model with _entity_descriptor. In get_mapper this seems to hit the "if isinstance(mixed, sa.Table):" branch and: if mixed in mapper.tables ends true to all 3 of them as it's either the parent or child that inherits it.

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[Mike Bayer]

get_mapper() is not part of SQLAlchemy where is it?

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[piotr maliński]

It's from sqlalchemy_utils  - https://sqlalchemy-utils.readthedocs.io/en/latest/orm_helpers.html#get-mapper

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[Mike Bayer]

OK then please report this as a bug in sqlalchemy-utils

right now they are not looking at local table

looking at the code here: https://sqlalchemy-utils.readthedocs.io/en/latest/_modules/sqlalchemy_utils/functions/orm.html#get_mapper

code can be like:

        mappers = [
            mapper for mapper in all_mappers
            if mixed in {mapper.local_table}
        ]

where above, using "table in set()" makes sure this compared on hash/eq rather than "is" to accommodate for Table vs. AnnotatedTable

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[piotr maliński]

That seems to help. I'll make an issue for it.

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