Prevent m2m_changed from firing when creating an object

Question

When using a Django signal like post_save you can prevent it from firing when an object is first created by doing something like:

@receiver(post_save,sender=MyModel)
def my_signal(sender, instance, created,**kwargs):
    if not created:
        pass # Do nothing, as the item is new.
    else:
        logger.INFO("The item changed - %s"%(instance) )

However, ManyToMany relations are applied after an item is initially created, so no such argument is passed in, making it difficult to suppress in these cases.

@receiver(m2m_changed,sender=MyModel.somerelation.though)
def my_signal(sender, instance, created,**kwargs):
    if __something__: # What goes here?
        pass # Do nothing, as the item is new.
    else:
        logger.INFO("The item changed - %s"%(instance) )

Is there an easy way to suppress an m2m_changed signal when its being done on an object that has just been created?

Answer 1

I think there is no easy way to do that.

As the Django doc says, you can't associate an item with a relation until it's been saved. Example from the doc:

>>> a1 = Article(headline='...')
>>> a1.publications.add(p1)
Traceback (most recent call last):
...
ValueError: 'Article' instance needs to have a primary key value before a many-to-many relationship can be used.

# should save Article first
>>> a1.save()
# the below statement never know it's just following a creation or not
>>> a1.publications.add(p1)

It's logically not possible for a relation record to know whether it is added to "a just created item" or "an item that already exists for some time", without external info.

Some workarounds I came up with:

Solution 1. add a DatetimeField in MyModel to indicate creation time. m2m_changed handler uses the creation time to check when is the item created. It work practically in some cases, but cannot guarantee correctness

Solution 2. add a 'created' attribute in MyModel, either in a post_save handler or in other codes. Example:

@receiver(post_save, sender=Pizza)
def pizza_listener(sender, instance, created, **kwargs):
    instance.created = created

@receiver(m2m_changed, sender=Pizza.toppings.through)
def topping_listener(sender, instance, action, **kwargs):
    if action != 'post_add':
        # as example, only handle post_add action here
        return
    if getattr(instance, 'created', False):
        print 'toppings added to freshly created Pizza'
    else:
        print 'toppings added to modified Pizza'
    instance.created = False

Demo:

p1 = Pizza.objects.create(name='Pizza1')
p1.toppings.add(Topping.objects.create())
>>> toppings added to freshly created Pizza
p1.toppings.add(Topping.objects.create())
>>> toppings added to modified Pizza

p2 = Pizza.objects.create(name='Pizza2')
p2.name = 'Pizza2-1'
p2.save()
p2.toppings.add(Topping.objects.create())
>>> toppings added to modified Pizza

But be careful using this solution. Since 'created' attribute was assigned to Python instance, not saved in DB, things can go wrong as:

p3 = Pizza.objects.create(name='Pizza3')
p3_1 = Pizza.objects.get(name='Pizza3')
p3_1.toppings.add(Topping.objects.create())
>>> toppings added to modified Pizza
p3.toppings.add(Topping.objects.create())
>>> toppings added to freshly created Pizza

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That's all about the answer. Then, caught you here! I'm zhang-z from github django-notifications group :)