I would suggest to read Association Proxy: Simplifying Association Objects. In this case your code would translate into something like below:
# NEW: need this function to auto-generate the PK for newly created Language
# here using uuid, but could be any generator
def _newid():
import uuid
return str(uuid.uuid4())
def _language_find_or_create(language_name):
language = Language.query.filter_by(language_name=language_name).first()
return language or Language(language_name=language_name)
class User(Base):
__tablename__ = 'user'
uid = Column(String(80), primary_key=True)
languages = relationship('Language', lazy='dynamic',
secondary='user_language')
# proxy the 'language_name' attribute from the 'languages' relationship
langs = association_proxy('languages', 'language_name',
creator=_language_find_or_create,
)
class UserLanguage(Base):
__tablename__ = 'user_language'
__tableargs__ = (UniqueConstraint('uid', 'lid', name='user_language_ff'),)
id = Column(Integer, primary_key=True)
uid = Column(String(80), ForeignKey('user.uid'))
lid = Column(String(80), ForeignKey('language.lid'))
class Language(Base):
__tablename__ = 'language'
# NEW: added a *default* here; replace with your implementation
lid = Column(String(80), primary_key=True, default=_newid)
language_name = Column(String(30))
# test code
user = User(uid="user-1")
# NEW: add languages using association_proxy property
user.langs.append("English")
user.langs.append("Spanish")
session.add(user)
session.commit()
user2 = User(uid="user-2")
user2.langs.append("English") # this will not create a new Language row...
user2.langs.append("German")
session.add(user2)
session.commit()