Declarative backref rels not filtering by polymorphic identity

[revmischa]

More info here:
https://bitbucket.org/zzzeek/sqlalchemy/issues/3960/declarative-backref-not-filtering-by

"the issue is somewhere in db.Model where their declarative base is interfering with declarative being able to set the "single inheritance" flag on the Event1 mapper."

[zzzeek]

likely because tablename is set to non-None in all cases now: https://github.com/mitsuhiko/flask-sqlalchemy/blob/master/flask_sqlalchemy/__init__.py#L641 that should be None to indicate single inheritance.

[revmischa]

@zzzeek it should be unset on the "base" classes, but set on the "child" classes? to the same thing?

[zzzeek]

whichever class should have its own table gets tablename, whichever subclass that's single inh either should not have tablename at all (declarative looks in cls.dict to check that the attribute is local to a certain class) or it should return None - declarative then sets that mapper as single=True.

Basically the way db.Model here wants to do tablename, if it has that feature then you need to set tablename = None on your model.

[davidism]

I'm working on fixing the tablename logic, I'll keep this example in mind.

[mark-anders]

Btw, the title of the bug is incorrect: it doesn't really involve relationships at all. It is that queries that specify a subclass do not emit a where clause when using polymorphic identity in single table, so effectively, the type is ignored.

Here's an example:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy import Column, Integer, String

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
db = SQLAlchemy(app)
app.config['SQLALCHEMY_ECHO'] = True

class Group(db.Model):
    __tablename__ = 'groups'

    group_id = Column(Integer, primary_key=True)
    name = Column(String)
    type = Column(String)

    __mapper_args__ = {
        'polymorphic_on': type,
        'polymorphic_identity': "group",
    }

    def __init__(self, name):
        self.name = name

class OwnerGroup(Group):
    # __tablename__ = None

    __mapper_args__ = {
        'polymorphic_identity': "owner"
    }


class SharedGroup(Group):
    # __tablename__ = None
    __mapper_args__ = {
        'polymorphic_identity': "shared"
    }

db.drop_all()
db.create_all()
session = db.session

o1 = OwnerGroup("#1 owner")
o2 = OwnerGroup("#2 owner")
s = SharedGroup("Shared")
session.add_all([o1,o2,s])
session.commit()
q = session.query(SharedGroup)
print(q)
found = q.all()
assert len(found) == 1, "Found too many SharedGroups"

Based on the hint from @zzzeek , if you uncomment the __tablename__ = None, it works.

This seems to be a pretty serious bug. Hope it gets fixed soon.

[davidism]

Duplicate of #478. Should be covered by #541, which also adds a test that the query contains the polymorphic condition.