title: How to use save_model in an AdminForm containing a M2M field?
body: <p>i'm having an annoying issue with django model system + its default admin.</p>
  
  <p>Let's assume i have a very simple model like:</p>
  
  <pre><code>class Note(models.Model):
      text = models.CharField(max_length=200)
  
  def __unicode__(self):
      return self.text
  </code></pre>
  
  <p>and a container like:</p>
  
  <pre><code>class NoteCollection(models.Model):
      notelist = models.ManyToManyField(Note)
      title = models.CharField(max_length=20)
  
  def __unicode__(self):
      return self.title
  </code></pre>
  
  <p>What i want do to it's update all the "Note" elements when a NoteCollection gets Added.
  I read that m2m models have complex save mechanism, so what i was thinking is, let's read the form object, and just save the Note elements by myself!!</p>
  
  <p>But when i make something like this in APPNAME/admin.py:</p>
  
  <pre><code>from models import Note,NoteCollection
  from django.contrib import admin
  
  class NoteCollectionAdmin(admin.ModelAdmin):
      def save_model(self, request, obj, form, change):
          # Something USEFUL HERE
          pass
  
  admin.site.register(Note)
  admin.site.register(NoteCollection, NoteCollectionAdmin)
  </code></pre>
  
  <p>Django pops me an error: ('NoteCollection' instance needs to have a primary key value before a many-to-many relationship can be used.)</p>
  
  <p><strong>I don't even want to use the NoteCollection object at all</strong>, i'm interested in the form object, actually..</p>
  
  <p>I've also found on internet some examples that use save_model with a M2M field, so i can't understand why i keep getting this error; for reference, i've just made a new-from-scrap project and i'm using an sqlite db for testing</p>