Question

I have a Django model with 2 many-to-many fields. When the model is saved from the admin interface I need to check if the second field is blank and if it is blank then I need the items from the first fielded copied to the second. How can I do this?

UPDATE

Matthew’s answer seems like it would work great but I can’t get the instance to save after I copied the field. I have tried instance.save() without any success.

Answer 1

The signal to be use is not post_save, rather m2m_changed, that is sent much after the model is saved to the database.

@models.signals.m2m_changed(sender=MyModel.second_m2m.through)
def duplicate_other_on_this_if_empty(sender, instance, action, reverse, model, pk_set, **kwargs):
    # just before adding a possibly empty set in "second_m2m", check and populate.
    if action == 'pre_add' and not pk_set:
        instance.__was_empty = True
        pk_set.update(instance.first_m2m.values_list('pk', flat=True))

@models.signals.m2m_changed(sender=MyModel.first_m2m.through)
def duplicate_this_on_other_if_empty(sender, instance, action, reverse, model, pk_set, **kwargs):
    # Just in case the "first_m2m" signals are sent after the other
    # so the actual "population" of the "second_m2m" is wrong:
    if action == 'post_add' and not pk_set and getattr(instance, '__was_empty'):
        instance.second_m2m = list(pk_set)
        delattr(instance, '__was_empty')

Edit: next code is simpler, and based upon new knowledge on the models definition

In your code the ‘first_m2m’ signals are sent before the the ‘second_m2m’ (it really depends on your model definition). So we can work on the assumption that when the ‘second_m2m’ signals are received, the ‘first_m2m’ is already populated with current data.

This makes us happier, because now you only need to check the m2m-pre-add:

@models.signals.m2m_changed(sender=MyModel.second_m2m.through)
def duplicate_other_on_this_if_empty(sender, instance, action, reverse, model, pk_set, **kwargs):
    # just before adding a possibly empty set in "second_m2m", check and populate.
    if action == 'pre_add' and not pk_set:
        pk_set.update(instance.first_m2m.values_list('pk', flat=True))