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

code

from sqlalchemy import create_engine
from sqlalchemy import Column, ForeignKey, Integer, String, Unicode, UniqueConstraint
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.orm.collections import attribute_mapped_collection


Base = declarative_base()

class Text(Base):
    __tablename__ = 'text'
    id = Column(Integer, primary_key=True)
    _values = relationship('TextValue', collection_class=attribute_mapped_collection('lang_id'))
    values = association_proxy('_values', 'value', creator=lambda k, v: TextValue(lang_id=k, value=v))


class Lang(Base):
    __tablename__ = 'lang'
    id = Column(String(4), primary_key=True)
    name = Column(String(20))


class TextValue(Base):
    __tablename__ = 'text_value'
    __table_args__ = (UniqueConstraint('lang_id', 'text_id'),)
    text_id = Column(Integer, ForeignKey(Text.id), primary_key=True)
    lang_id = Column(Integer, ForeignKey(Lang.id), primary_key=True)
    value = Column(Unicode)

engine = create_engine('sqlite:////tmp/test.db')
Base.metadata.create_all(engine)
Session = sessionmaker()
Session.configure(bind=engine)
session = Session()

values = {'pl': u'org', 'en': u'org'}
session.add(Text(values=values))
text = session.query(Text).one()
text.values = {'pl': u'modified', 'en': u'modified'}
session.commit()

traceback

Traceback (most recent call last):
  File "/home/piotr/.PyCharm50/config/scratches/scratch_20", line 41, in <module>
    session.commit()
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 813, in commit
    self.transaction.commit()
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 392, in commit
    self._prepare_impl()
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 372, in _prepare_impl
    self.session.flush()
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2027, in flush
    self._flush(objects)
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2145, in _flush
    transaction.rollback(_capture_exception=True)
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2109, in _flush
    flush_context.execute()
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 373, in execute
    rec.execute(self)
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 487, in execute
    self.dependency_processor.process_saves(uow, states)
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/dependency.py", line 558, in process_saves
    uowcommit, False)
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/dependency.py", line 575, in _synchronize
    sync.clear(dest, self.mapper, self.prop.synchronize_pairs)
  File "/home/piotr/.virtualenvs/pab/lib/python2.7/site-packages/sqlalchemy/orm/sync.py", line 74, in clear
    (r, orm_util.state_str(dest))
AssertionError: Dependency rule tried to blank-out primary key column 'text_value.text_id' on instance '<TextValue at 0x7f6b260d9710>'

Process finished with exit code 1