How to set column_property?

Question

Flask

It doesn't work like that

user_count = column_property(
        db.session.query(func.count(User.id))
    )
# RuntimeError: application not registered on db instance and no application bound to current context

There are generally some problems with mapping. The error is not related to this table at all.

user_count = column_property(
        User.query.with_entities(func.count(User.id))
    )

This is how it works

user_count = column_property(
        select([func.count(User.id)])
    )

But here is the problem with the jane

user_count = column_property(
        select([func.count(User.id)])
             .join(User.attrs)
    )
# sqlalchemy.exc.ArgumentError: FROM expression expected

Why does model query not work and what to do with select?