Question

I have found this code http://djangosnippets.org/snippets/2283/ but I think it doesn’t work with manytomanyfields which use another model (keyword through) – I get an AttributeError when trying to merge.

Do you know any way to fix this or another method for merging objects ?

Edit: more details

I have 3 models: A, B, C

A has a manytomany field “m2mfield” pointing to C through B.

When I run the code from django snippets, it fails with the exception

'ManyRelatedManager' object has no attribute 'remove'

I think this has something to do with a comment in Django source (django.db.models.fields.related.py line 499) which says:

# If the ManyToMany relation has an intermediary model,
# the add and remove methods do not exist.

I think the code snippet I got doesn’t make any difference between ManyToMany relations with and without intermediary model. That’s why I’m looking for some way to fix that code or another way to achieve what I want (merging).

Answer 1

I eventually modified the code to handle the case of ManyToMany fields created ‘through’ a model.
Here is what is to be modified:

# Migrate all many to many references from alias object to primary object.
for related_many_object in alias_object._meta.get_all_related_many_to_many_objects():
    alias_varname = related_many_object.get_accessor_name()
    obj_varname = related_many_object.field.name

    # Treatment depends on if the many_to_many field is created through another model
    if getattr(alias_object, alias_varname).through._meta.auto_created:
        if alias_varname is not None:
            # standard case
            related_many_objects = getattr(alias_object, alias_varname).all()
        else:
            # special case, symmetrical relation, no reverse accessor
            related_many_objects = getattr(alias_object, obj_varname).all()
        for obj in related_many_objects.all():
            getattr(obj, obj_varname).remove(alias_object)
            getattr(obj, obj_varname).add(primary_object)
    else:
        related_many_objects = getattr(alias_object, alias_varname).all()

        through_model = getattr(alias_object, alias_varname).through
        through_field_name = None
        for f in through_model._meta.fields:
            if isinstance(f, ForeignKey):
                if f.rel.to == primary_class :
                # f is the field in our 'through' model which points to an instance of primary_class
                    through_field_name = f.name

        for obj in related_many_objects.all():
            kwargs = {
                through_field_name: obj,
            }
            for through_obj in through_model.objects.filter(**kwargs):
                setattr(through_obj, through_field_name, primary_object)
                through_obj.save()