Django loop over all m2m relations - django

Question

I need to loop over all m2m relations of the model instance and copy them to new model instance.
source_id=request.GET.get('source_id', 1)
obj = Artist.objects.create(title='New artist')
source_obj = Artist.objects.get(id=source_id)
if source_obj.galleries.count():
obj.galleries = source_obj.galleries.all()
if source_obj.suggested_artists.count():
obj.suggested_artists = source_obj.suggested_artists.all()
Currently i am doing it like this, but i want to loop over all m2m fields and copy the related data to obj.
I want something like:
for m2m_rel in source_obj.m2m_relations:
print geattr(source_obj, m2m_rel).count()
print geattr(source_obj, m2m_rel).all()
Any suggestions?

Answer 1

You can access the m2m relation entries like this:
for field in source_obj._meta.many_to_many:
source = getattr(source_obj, field.attname)
for item in source.all():
# do something with item...
print repr(item)
If you're trying to clone a model intance, you can use a generic clone_objects function like the one below. The function will clone a list of objects and return a new list of cloned objects (with new ID's):
# import Python's copy library
import copy
def clone_objects(objects):
"""
Generic model object cloner function.
"""
def clone(obj):
"""Return an identical copy of the instance with a new ID."""
if not obj.pk:
raise ValueError('Instance must be saved before it can be cloned.')
duplicate = copy.copy(obj)
# Setting pk to None tricks Django into thinking this is a new object.
duplicate.pk = None
duplicate.save()
# ... but the trick loses all ManyToMany relations.
for field in obj._meta.many_to_many:
source = getattr(obj, field.attname)
destination = getattr(duplicate, field.attname)
for item in source.all():
destination.add(item)
return duplicate
if not hasattr(objects,'__iter__'):
objects = [ objects ]
objs = []
for obj in objects:
new_obj = clone(obj)
new_obj.save()
objs.append(new_obj)
return objs
The main part of the "cloning" code is from this snippet: Clone model mixin