Pierre B <rocambolesque...@gmail.com> wrote:
> Hi all, > > I'm ultimately trying to have different default values for the same column. > Following the documentation, the @declared_attr.cacading decorator seems to > be the best approach. > Here's my code: > class HasSomeAttribute(object): > @declared_attr.cascading > def type(cls): > if has_inherited_table(cls): > if cls.__name__ == 'MySubClass1': > return db.Column(db.Integer, default=1) > else: > return db.Column(db.Integer, default=2) > else: > return db.Column(db.Integer, default=0) > > class MyClass(HasSomeAttribute, db.Model): > __tablename__ = 'people4l2' > id = db.Column(db.Integer, primary_key=True) > > class MySubClass1(MyClass): > pass > > class MySubClass2(MyClass): > pass > > I iterated quite a few times over this but I'm systematically getting this > error: > ArgumentError: Column 'type' on class <class '__main__.MySubClass1'> > conflicts with existing column 'people4l2.type’ this mapping illustrates MySubClass1 and MySubClass2 as both sharing the same table “people4l2”, as they have no __tablename__ attribute, so there can only be one “type” column. So in this case it is not appropriate to use cascading in exactly this way, as MyClass already has a “type” column, and that gets attached to the “people4l2” table and that’s it; there can be no different “type” column on MySubClass1/MySubClass2. If you’d like “type” to do something different based on which class is being instantiated, this is an ORM-level differentiation. Use either the constructor __init__() to set it or use the init() event (http://docs.sqlalchemy.org/en/rel_0_9/orm/events.html?highlight=event%20init#sqlalchemy.orm.events.InstanceEvents.init). OTOH if “type” is actually the “polymoprhic discriminator”, which is what this looks like, then you’d be looking to just set up “type” as the “polymorphic_on” column and set up the “1”, “2”, “0” as the polymorphic identity (see http://docs.sqlalchemy.org/en/rel_0_9/orm/inheritance.html#single-table-inheritance for a simple example). -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.
