Detecting modifications in ManyToMany field

[Fabien Schwob]

Hello,

I would like to detect changes in a ManyToMany field, to update data
in the linked Model. I've two objects :

class Category(models.Model):
    .... some fields ...


class Post(models.Model):
    .... some fields ...
    categories = models.ManyToManyField(Category)

And I would like to detect (in the admin and when accessing the object
in python) when categories are added or removed to a Post. To update
some fields in the Category model. But I've tried with :

 * django.db.models.signals.m2m_changed, but it doesn't appear to be
trigged in the admin
 * by overriding Post.save(), but I get the same information before
and after the save() (I think it's because m2m are handled after
save() has finished) :

def save(self, *args, **kwargs):
        print Post.objects.get(pk=self.id).categories.all()
        super(Post, self).save(*args, **kwargs)
        print Post.objects.get(pk=self.id).categories.all()

Any solutions to handle this case ? With a custom signal, or anything else ?

Thanks

-- 

Fabien Schwob

[Vijay Khemlani]

You might find the m2m_changed signal useful

https://docs.djangoproject.com/en/1.7/ref/signals/#m2m-changed

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[Vijay Khemlani]

You might find the m2m_changed signal useful

https://docs.djangoproject.com/en/1.7/ref/signals/#m2m-changed

On Sat, Mar 7, 2015 at 7:57 AM, Fabien Schwob <xphu...@gmail.com> wrote:

--

[Fabien Schwob]

With the m2m changed signal I can only have the state of the object after it has been saved. Not the difference between before and after the change.

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[Vijay Khemlani]

Haven't tested it, but the "pre_add" and "pre_remove" actions in the m2m_changed signal should be triggered before the change, and they include a pk_set arg with the id's of the related M2M model being added / removed.

You could also link to the post_save and post_remove which contain the same flag, but that depends on what are you trying to accomplish.

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