SQLAlchemy Polymorphic Relationship with Concrete Inheritance

Question

I am using the concrete table inheritance with SQLAlchemy. In declartive style model class, I have configured it successfully.

My code just like:

class Entry(AbstractConcreteBase, db.Model):
    """Base Class of Entry."""

    id = db.Column(db.Integer, primary_key=True, nullable=False)
    created = db.Column(db.DateTime, nullable=False)
    post_id = declared_attr(lambda c: db.Column(db.ForeignKey("post.id")))
    post = declared_attr(lambda c: db.relationship("Post", lazy="joined"))

    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

    @declared_attr
    def __mapper_args__(cls):
        # configurate subclasses about concrete table inheritance
        return {'polymorphic_identity': cls.__name__,
                'concrete': True} if cls.__name__ != "Entry" else {}

class TextEntry(Entry):
    """Text and Article Entry."""

    text = db.deferred(db.Column(db.Text, nullable=False))

class PhotoEntry(Entry):
    """Photo Entry."""

    path = db.deferred(db.Column(db.String(256), nullable=False))

It works fine while testing it in the shell:

>>> from models.entry import Entry
>>>
>>> Entry.query.all()
[<PhotoEntry 'Title' created by tonyseek>,
 <PhotoEntry 'TITLE 2' created by tonyseek>,
 <PhotoEntry 'Title 3' created by tonyseek>,
 <PhotoEntry 'Title 4' created by tonyseek>,
 <TextEntry 'Title' created by tonyseek>]

Then I fall into trouble while setting the relationship in other models. Each entry has a foreign key post_id to join Post model, but I could not define the back reference in Post. That can't work:

class Post(db.Model):
    """An Post."""

    id = db.Column(db.Integer, primary_key=True, nullable=False)
    description = db.Column(db.Unicode(140), nullable=False)
    entries = db.relationship(Entry, lazy="dynamic")

It raised a Exception and said:

InvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers. Original exception was: Class 'models.entry.Entry' is not mapped.

Obvious the Entry is a abstract class, which couldn't be mapped to a real exist table. The document in official website has a example but its base class is not abstract. Now how should I do to set the polymorphic relationship with a abstract model?

Answer 1

I have found the reason of the problem and its solution.

According to the document of sqlalchemy offical website, the abstract class could be a mapped class, because the polymorphic_union function could create a virtual table.

I am using the declartive style model, not build mapper by hand, so the virtual table pjoin should not be created by hand. The base class AbstractConcreteBase has a method __delcare_last__ would create the pjoin with polymorphic_union function, but it would be called while the event after_configured triggering.

The relationship with Entry in Post would be created after the Post class be generated, in this time the event after_configured have not been triggered, so __delcare_last__ function have not created the virtual table pjoin and mapped it into Entry. So the exception "Class 'models.entry.Entry' is not mapped." will be raised.

Now, I refactor the Post model, let it create the relationship with Entry in __delcare_last__ function, then it will be success because of the triggered event and the mapped Entry.

My new implemented class like this:

class Post(db.Model):
    """An Post."""

    id = db.Column(db.Integer, primary_key=True, nullable=False)
    description = db.Column(db.Unicode(140), nullable=False)

    @classmethod
    def __declare_last__(cls):
        cls.entries = db.relationship(Entry, viewonly=True)

    def attach_entries(self, entries):
        """Attach Entries To This Post.

        Example:
            >>> post = Post("An Interesting News", "Wow !!!")
            >>> text_entry = TextEntry(*t_args)
            >>> photo_entry = PhotoEntry(*p_args)
            >>> post.attach_entries([text_entry, photo_entry])
            >>> len(post.entries)
            2
            >>> db.session.commit()
            >>>
        """
        for entry in entries:
            self.entries.append(entry)
            entry.post = self
            db.session.add(entry)