[Django]-Django onetoonefield not saving instance

At first create B instance, assign attributes to it, call save on it (now it have id) and then assign it to mashup.b:

if cr:
     b = B()
else:
    if mashup.b is None:
        b = B()

#.... (assign values to b attributes)

b.save()
mashup.b = b
mashup.save()