title: Adding inline many to many objects in Django admin
body: <p>I'm fairly new to Django and having read the documentation on its relational models and inline admin forms (<a href="https://docs.djangoproject.com/en/dev/ref/contrib/admin/#inlinemodeladmin-objects">docs on InlineModelAdmin</a>) I'm struggling to figure out if the following is possible out of the box, or if I should roll my own forms.</p>
  
  <p>Let's say I have two objects: Films and Directors, this is a many-to-many relationship as defined in in the model declarations as follows:</p>
  
  <pre><code>class Film(Model):
      director = ManyToManyField('Director')
  </code></pre>
  
  <p>Now in the detail form for a Film object I would like to <em>add</em> inline Director objects (they just have a name field as sole property). Not just selecting existing instances, but being able to <em>create new ones</em>, inline in the form of the Film object. </p>
  
  <pre><code>class DirectorInline(admin.TabularInline):
      model = Director
      extra = 3
  
  
  class FilmAdmin(admin.ModelAdmin):
      inlines = (
          DirectorInline,
          )
  </code></pre>
  
  <p>This throws an error, because it expects a foreign key on the Director object. 
  Is what I'm trying to achieve possible without creating a custom form, validator etc. ?
  Any tips in the right direction would be greatly appreciated, thanks in advance.</p>