title: Django : save a new value in a ManyToManyField
body: <p>I gave details on my code : I don't know why my table is empty (it seems that it was empty out after calling <code>save_model</code>, but I'm not sure).</p>
  
  <pre><code>class PostAdmin(admin.ModelAdmin):
      def save_model(self, request, post, form, change):
          post.save()
  
          # Authors must be saved after saving post
          print form.cleaned_data['authors'] # []
          print request.user # pg
          authors = form.cleaned_data['authors'] or request.user
          print authors # pg
          post.authors.add(authors)
  
          print post.authors.all() # [&lt;User: pg&gt;]
  
          # But on a shell, table is empty. WTF ?! : 
          # select * from journal_post_authors;
          # Empty set (0.00 sec)
  </code></pre>