Instantiate django model from parent data

Question

Hi,

Is it possible to instantiate a subclassed model from its parent?

class Object1(models.Model):
  field1a = models.CharField()
  field1b = models.CharField()
  feild1c = models.ForeignKey(Object4)      

class Object2(Object1):
  field3 = models.CharField()

class Object3(Object1):
  field3 = models.CharField()

class Object4(models.Model):
  field4 = models.CharField()

What I want to do is create the base class first and then based on some rule instantiate one of the subclasses but using the already created base class.

Something like:

obj4 = Object4(field4='d')
obj1 = Object1(field1a='a', field1b='b', field1c=obj4)
if somerule:
  obj2 = Object2(object1_ptr=obj1, field2='2')
else:
  obj3 = Object3(object1_ptr=obj1, field3='3')

I don't want to repeat the Object1 fields in the if/else clauses. Is it possible to accomplish this? When I try this I get a Foreign key error; Cannot add or update a child row: A foreign key constraint fails

Answer 1

I recommend doing something like this:

attr = dict(field1a='a', field1b='b', field1c=obj4)
obj1 = Object1(**attr)
if somerule:
    attr["field2"] = 2
    obj2 = Object2(**attr)
else:
    attr["field3"]='3'
    obj3 = Object3(**attr)

Be aware that the dictionary attr changes in place.

Answer 2

What you're doing is almost correct, but if you want to copy it you will have to remove the primary key.

So... this should fix it: del obj2.id Do note however that if some other model references your model with a foreign key that it references obj1, not obj2. And obj1 will, ofcourse, still exist.