SQLAlchemy: Delete object directly from one-to-many relationship without using session.delete()

Question

I have the following SQLAlchemy setup:

Base = declarative_base()

class Post(Base):
    __tablename__ = 'post'
    id = Column(Integer, primary_key=True)
    title = Column(String(30))
    comments = relationship('Comment', cascade='all')

class Comment(Base):
    __tablename__ = 'comment'
    id = Column(Integer, primary_key=True)
    post_id = Column(Integer, ForeignKey(Post.id, ondelete='CASCADE'), nullable=False)
    text = Column(Text)

With this, I can create post objects with a one-to-many relationship to comments. I want to handle creation and deletion of comments for posts without referencing the session. Adding a comment to a post works just fine:

post = Post(title='some post')
comment = Comment(text='some comment')
post.comments.append(comment)

My session handler only knows about posts, so it would do a session.add(post) and the comment is placed into the session automatically and is syncronized with the database on the next session.commit(). However, the same is not true for deletion of comments. I want to be able to delete a comment by just doing:

post.comments.remove(comment)

However, this produces the following error on the next session.commit():

sqlalchemy.exc.OperationalError: (OperationalError) (1048, "Column 'post_id' cannot be null") 'UPDATE comment SET post_id=%s WHERE comment.id = %s' (None, 1L)

How do I tell SQLAlchemy to not update the comment with a NULL value for post_id (which is not allowed due to the not null constraint on the column), but delete the comment instead? I know that I could do session.delete(comment), but since I did not need to add the comment to the session explicitly, I don't see a reason why I should have to delete it from the session explicitly.

I found several solutions for cascading deletes to related objects, but since I never issue any explicit deletions to the session (the post is still there), I don't think that is applicable.

Edit: I adjusted the example to include the cascading of deletions from posts. Now it works to do session.delete(post) and all comments are deleted. But I just want to automatically delete the comments I removed from the relationship and not delete the whole post with all comments.

TL;DR: How do I tell SQLAlchemy to issue a delete statement instead of an update statement when I remove an entry from a relationship list of a one-to-many relationship?

Answer 1

Read Configuring delete/delete-orphan Cascade section of documentation for more information, but basically you need delete-orphan as well in your cascade option of the relationship:

class Post(Base):
    # ...
    comments = relationship('Comment', cascade="all, delete-orphan")

Answer 2

I am not a SQLAlchemy user, but I think you should use the ondelete option

    post_id = Column(Integer, ForeignKey(Post.id, ondelete="CASCADE"), nullable=False)

See, Mysql 5.6 Manual 13.6.44, FOREIGN KEY Constraints

SET NULL: Delete or update the row from the parent table, and set the foreign key column or columns in the child table to NULL.
Both ON DELETE SET NULL and ON UPDATE SET NULL clauses are supported.

CASCADE: Delete or update the row from the parent table, and automatically delete or update the matching rows in the child table.
Both ON DELETE CASCADE and ON UPDATE CASCADE are supported. Between two tables, do not define several ON UPDATE
CASCADE clauses that act on the same column in the parent table or in the child table.

And http://docs.sqlalchemy.org/en/rel_0_9/core/constraints.html Section: Defining Foreign Keys -> ON UPDATE and ON DELETE