Automatically create related model

Question

With SQLAlchemy I have a 1-1 relationship:

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)

class UserProfile(Base):
    __tablename__ = 'user_profiles'
    user_id = Column(Integer, ForeignKey(User.id), primary_key=True)
    user = relationship(User, 
                        backref=backref('profile', uselist=False), 
                        foreign_keys=did)

It's required that every User always has an associated UserProfile, even if created like this:

user = User()
session.add(user)
session.commit(user)

Is there a way to automatically create and associate a related entity?

Currently, I'm doing it this way:

@event.listens_for(User, 'init')
def on_user_init(target, args, kwargs):
    if not target.profile:
        target.profile = UserProfile()

However this sometimes results in

IntegrityError: (IntegrityError) duplicate key value violates unique constraint "user_profile_pkey" DETAIL: Key (user_id)=(1) already exists. 'INSERT INTO user_profiles ...

since UserProfile is assigned to a User which already exists.

ORM Event "before_insert" is not applicable since the docs clearly state it is not allowed to add new or modify current instances.

Any better way to achieve that?

kolypto · Accepted Answer · 2014-09-08 08:14:05Z

1

The quick-and-easy way is to use "init" event, but filter out instances which have a primary key. Note that at the moment of instance initialization, it's empty, and kwargs argument contains the fields SqlAlchemy (or your code) is going to set on it:

@event.listens_for(User, 'init')
def on_user_init(target, args, kwargs):
    if not target.profile and 'id' not in kwargs:
        target.profile = UserProfile()

This still won't work when you save instances with id set manually (e.g. User(id=1)), but will cover most of other cases.

As for now, I don't see a better way.

answered Sep 8, 2014 at 8:14

kolypto

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