Django Many-to-Many relation insertion control

Question

I have the following models:

class Item(models.Model):
    # fields
    # ...

class Collection(models.Model):
    ite         


        

Answer 1

1. The pre_save signal is called before saving an instance. But you are not able to abort the save operation from there. A better solution would be to add a new method to your Collection model, which is responsible for checking if an Item can be added:

   class Collection(models.Model):
       items = models.ManyToManyField(Item, related_name="collections")
   
       ...
   
       def add_item(self, item):
           if check_if_item_can_be_added(item):
               items.add(item)
               self.save()
   
   def check_if_item_can_be_added(self, item):
       # do your checks here
   

2. When adding an instance to a m2m field, the save method does not get called. You are right, the m2m_changed signal is the way to go. You can safely update the collection instance in there.