fixing "assignment to the reverse side of a many-to-many.."

[matinmidietsoir]

in extend.py add_bidirectional_m2m

fixing the bug : "direct assignment to the reverse side of a many-to-many set is prohibited.
I use what I found here https://gist.github.com/Wtower/0b181cc06f816e4feac14e7c0aa2e9d0

sorry if my procedure isn't correct, it's my first pull request :).

        def save(self, commit=True):
            """
            Saves this ``form``'s cleaned_data into model instance
            ``self.instance``.

            If commit=True, then the changes to ``instance`` will be saved to the
            database.  If ``instance`` is a new object then it will get saved to
            the database even if commit=False

            Returns ``instance``.
            """
            instance = super(BidirectionalM2MForm, self).save(commit=False)
            force_save = self.instance.pk is None
            if force_save:
                instance.save()
            for m2m_field, related_manager in self._get_bidirectional_m2m_fields():

#                setattr(self.instance, related_manager, self.cleaned_data[m2m_field])
#   direct assignment to the reverse side of a many-to-many set is prohibited. Use suppliers.set() instead.

                recordset = getattr(self.instance, m2m_field)
                records = recordset.all()
                # remove records that have been removed in form
                for record in records:
                    if record not in self.cleaned_data[m2m_field]:
                        recordset.remove(record)
                # add records that have been added in form
                for record in self.cleaned_data[m2m_field]:
                    if record not in records:
                        recordset.add(record)

            if commit:
                if not force_save:
                    instance.save()
                self.save_m2m()
            return instance

    return BidirectionalM2MForm