[Django]-Issue with ManyToMany Relationships not updating immediately after save

When you are trying to access the ManyToMany fields in the post_save signal of the model, the related objects have already been removed and will not be added again until after the signal is finished.

To access this data you have to tie into the save_related method in your ModelAdmin. Unfortunately you’ll also have to include the code in the post_save signal for non-admin requests that require customization.

see: https://docs.djangoproject.com/en/1.7/ref/contrib/admin/#django.contrib.admin.ModelAdmin.save_related

Example:

# admin.py
Class GroupAdmin(admin.ModelAdmin):
    ...
    def save_related(self, request, form, formsets, change):
        super(GroupAdmin, self).save_related(request, form, formsets, change)
        # do something with the manytomany data from the admin
        form.instance.users.add(some_user)

Then in your signals you can make the same changes that you want to execute on a save:

# signals.py
@receiver(post_save, sender=Group)
def group_post_save(sender, instance, created, **kwargs):
    # do something with the manytomany data from non-admin
    instance.users.add(some_user)
    # note that instance.users.all() will be empty from the admin: []

I have a general solution to this that seems a bit cleaner than monkey-patching the core or even using celery (although I’m sure someone could find areas where it fails). Basically I add a clean() method in the admin for the form that has the m2m relationships, and set the instance relations to the cleaned_data version. This make the correct data available to the instance’s save method, even though it’s not “on the books” yet. Try it and see how it goes:

def clean(self, *args, **kwargs):
    # ... actual cleaning here
    # then find the m2m fields and copy from cleaned_data to the instance
    for f in self.instance._meta.get_all_field_names():
        if f in self.cleaned_data:
            field = self.instance._meta.get_field_by_name(f)[0]
            if isinstance(field, ManyToManyField):
                setattr(self.instance,f,self.cleaned_data[f])

See http://gterzian.github.io/Django-Cookbook/signals/2013/09/07/manipulating-m2m-with-signals.html

problem:
When you manipulate the m2m of a model within a post or pre_save signal receiver, your changes get wiped out in the subsequent ‘clearing’ of the m2m by Django.

solution:
In you post or pre_save signal handler, register another handler to the m2m_changed signal on the m2m intermediary model of the model whose m2m you want to update.

Please note that this second handler will receive several m2m_changed signals, and it is key to test for the value of the ‘action’ arguments passed along with them.

Within this second handler, check for the ‘post_clear’ action. When you receive a signal with the post_clear action, the m2m has been cleared by Django and you have a chance to successfully manipulate it.

an example:

def save_handler(sender, instance, *args, **kwargs):
    m2m_changed.connect(m2m_handler, sender=sender.m2mfield.through, weak=False)


def m2m_handler(sender, instance, action, *args, **kwargs):
    if action =='post_clear':
        succesfully_manipulate_m2m(instance)


pre_save.connect(save_handler, sender=YouModel, weak=False)

see https://docs.djangoproject.com/en/1.5/ref/signals/#m2m-changed

It would help if you use on_commit, so ManyToMany Relationships will be updated.

class Publication(models.Model):
    title = models.CharField(max_length=30)

class Article(models.Model):
    publications = models.ManyToManyField(Publication)

@receiver(post_save, sender=Article)
def my_signal(sender, **kwargs):
    from django.db import transaction
    transaction.on_commit(
        lambda: do_something_after_commit()
    )

    def do_something_after_commit():
        # Here you have access to updated m2m fields
        pass

You can find more informations in this thread : Django manytomany signals?

One of the solutions to update m2m, along with updating one of your models.

Django 1.11 and higher

First of all, all requests via admin panel are atomic. You can look at ModelAdmin:

@csrf_protect_m
def changeform_view(self, request, object_id=None, form_url='', extra_context=None):
    with transaction.atomic(using=router.db_for_write(self.model)):
        return self._changeform_view(request, object_id, form_url, extra_context)

@csrf_protect_m
def delete_view(self, request, object_id, extra_context=None):
    with transaction.atomic(using=router.db_for_write(self.model)):
        return self._delete_view(request, object_id, extra_context)

The behavior which you can observe during updating, when changes which you made with m2m records were not saved, even after you made them in a save method one of your models or in a signal, happens only because m2m form rewrites all records after the main object is updated.

This is why, step by step:

1. The main object is updated.

2. Your code(in a save method or in a signal) made changes (you can look at them, just put a breakpoint in ModelAdmin):

 def save_related(self, request, form, formsets, change):
     breakpoint()
     form.save_m2m()
     for formset in formsets:
         self.save_formset(request, form, formset, change=change)

3. form.save_m2m() takes all m2m values which were placed on a page(roughly speaking) and replace all m2m records via a related manager. That’s why you can’t see your changes at the end of a transaction.

There is a solution: make your changes with m2m via
transaction.on_commit. transaction.on_commit will make your changes
after form.save_m2m() when the transaction is committed.

Unfortunately, the downside of this solution – your changes with m2m will be executed in a separate transaction.