Hi there, Well, just try to make sure you are passing already saved instance (having primary key set) into related model which has foreign key to the first one.
Your approach: > from test.models import A,B > instA = A() > instB = B(a=instA) > > Now I'm done with my parsing and am happy to save them to my database: > > instA.save() # all good > instB.save() # fails with IntegrityError: null value in column "a_id" > violates not-null constraint Could easily be fixed: > from test.models import A,B > instA = A() > #instB = B(a=instA) # We will do that later > > Now I'm done with my parsing and am happy to save them to my database: > > instA.save() # all good > instB = B(a=instA) # Now is the time > instB.save() # shouldn't raise any exception This is common problem if you are trying to get use to ORM. I had many problems with understanding Hibernate/JPA a while ago when coding in Java. IntegrityError was risen because instA was cached at instB (you may check this by comparing instB.a.id and instB.__dict__), and changes in instA haven't been pushed. It's common approach for Object-Relation Mappers. It's probably tempting to prepare your objects first but preparing your data before is even better. If there would be not too much confusion you can try to use dicts as data storage first, then just pass it to the constructors of your models - just a note, I do this quite frequently in some cases. But if you are not convinced and you really have to prepare objects first you may simply set instA to instB just before calling ``save`` method at instB. I would be: > from test.models import A,B > instA = A() > instB = B() # prepare but don't set instA yet > > Now I'm done with my parsing and am happy to save them to my database: > > instA.save() # all good > instB.a = instA # now we set instA, *after* it has proper pk already set > instB.save() # shouldn't raise any exception Hope it helps On Dec 26, 2009, at 7:12 PM, gmayer wrote: > Hi guys, > > I'm new to Django but otherwise quite a seasoned python and sql > programmer. I'm baffled by django's foreign key id behaviour and as a > result stuck in my current coding project. Let me start immediately > with an example of what I'm trying to do. Assume two very simple > models: > > class A(models.Model): > pass > class B(models.Model): > a = models.ForeignKey(A) > > Now I need to create an instance of A and B without saving either of > them (I'm parsing a whack load of data and only want to commit objects > to the db once successfully parsed): > > from test.models import A,B > instA = A() > instB = B(a=instA) > > Now I'm done with my parsing and am happy to save them to my database: > > instA.save() # all good > instB.save() # fails with IntegrityError: null value in column "a_id" > violates not-null constraint > > On closer inspection the second line above fails because instB.a_id is > None, even though instB.a.id is defined. Why does django throw in a > duplicate, now stale instB.a_id field which, to make matters worse, is > used in generating the underlying SQL for instB.save() to result in > failure???? > > Perhaps I'm just to n00b to use the correct semantics which avoid the > above issues... excuse me if that's the case. > > Gunther > > -- > > You received this message because you are subscribed to the Google Groups > "Django users" group. > To post to this group, send email to django-us...@googlegroups.com. > To unsubscribe from this group, send email to > django-users+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/django-users?hl=en. > > -- You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-us...@googlegroups.com. To unsubscribe from this group, send email to django-users+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-users?hl=en.
