I can\'t find it anywhere, so your help will be nice for me :) Here is that field:
categories = models.ManyToManyField(fragmentCategory)
Fr
There's a whole page of the Django documentation devoted to this, well indexed from the contents page.
As that page states, you need to do:
my_obj.categories.add(fragmentCategory.objects.get(id=1))
or
my_obj.categories.create(name='val1')