AssertionError: Dependency rule tried to blank-out primary key column

[manc1]

Hello, I have a problem with nested that have a foreign key which is also a primary key. What I'm trying to accomplish is extremely simple.

Base: Any = declarative_base()
Session: Any = orm.scoped_session(orm.sessionmaker())

class Author(Base):
    __tablename__="authors"
    name = Column(String, primary_key=True)
    desc = Column(String, nullable=False)
    book_id = Column(Integer, ForeignKey("books.book_id"), primary_key=True)


class AuthorSchema(ModelSchema):

    class Meta:
        model = Author
        sqla_session = Session


class Book(Base):
    __tablename__ = "books"
    book_id = Column(Integer, primary_key=True)
    authors = relationship("Author", backref="book")


class BookSchema(ModelSchema):
    authors = fields.Nested(AuthorSchema, many=True)

    class Meta:
        model = Book
        sqla_session = Session

This is quite straightforward. I would like to have a book instance linked to multiple authors. Then, suppose I loads a JSON ponting to an existent book instance and suppose I would like to change desc value of an author. I then commit the change and get an AssertionError. Let`s see the code:

    engine = create_engine("postgresql://localhost", echo=False)
    Base.metadata.create_all(engine)
    Session.configure(bind=engine)
    session = Session()
    book = Book(book_id=6934389)
    author1 = Author(name="myname", desc='mydesc')
    book.authors = [author1]
    session.add(book)
    session.commit()
    schema = BookSchema()
    json_book = schema.dumps(book)
    schema = BookSchema()
    book = schema.loads(json_book)
    book.authors[0].desc = 'newdesc'
    session.add(book)
    session.commit()
    session.close()

Result

Traceback (most recent call last):
  File "/models/db_models.py", line 264, in <module>
    session.commit()
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 1027, in commit
    self.transaction.commit()
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 494, in commit
    self._prepare_impl()
  File ".venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 473, in _prepare_impl
    self.session.flush()
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 2459, in flush
    self._flush(objects)
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 2597, in _flush
    transaction.rollback(_capture_exception=True)
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 68, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 153, in reraise
    raise value
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 2557, in _flush
    flush_context.execute()
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/unitofwork.py", line 422, in execute
    rec.execute(self)
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/unitofwork.py", line 540, in execute
    self.dependency_processor.process_saves(uow, states)
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/dependency.py", line 584, in process_saves
    state, child, None, True, uowcommit, False
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/dependency.py", line 604, in _synchronize
    sync.clear(dest, self.mapper, self.prop.synchronize_pairs)
  File "/.venv/lib/python3.7/site-packages/sqlalchemy/orm/sync.py", line 87, in clear
    "column '%s' on instance '%s'" % (r, orm_util.state_str(dest))
AssertionError: Dependency rule tried to blank-out primary key column 'authors.book_id' on instance '<Author at 0x10f9a3190>'

Any idea of how can I solve this?

Thank you very much!

[eino]

I have the same error: "AssertionError: Dependency rule tried to blank-out primary key column", while loading a json data for an existing object. The error doesn't even occur when doing a session.add / session.merge, but directly when an autoflush is generated inside the load. The object whose primary key is being deleted is present in the json, so I guess the autoflush is generated for a partial data. Anyway it makes no sense to try to wipe the primary key (juste like your example). Especially as it's just in the load. It's very annoying.

[giovanni-bellini-argo]

i get the same error, any news on this? 👀