models.Model.save() method improvement.

[aki33524]

class Foo(models.Model):
  pass

class Bar(models.Model):
  foo = models.ForeignKey(Foo)

foo = Foo()
bar = Bar(foo=foo)

-------------------------
bar.save() <- Fail

-------------------------
foo.save()
bar.save() <- Fail

-------------------------
foo.save()
bar.foo = bar.foo
bar.save() <- Success!

This is very odd behavior.

And, when implementing foo.save() without foo.save(), I wish Django implemented foo.save() implicitly.
Like QuerySet.delete(), ON DELETE CASCADE emulation.