title: django admin inline many to many custom fields
body: <p>Hi I am trying to customize my inlines in django admin.</p>
  
  <p>Here are my models:</p>
  
  <pre><code>class Row(models.Model):
      name = models.CharField(max_length=255)
  
  class Table(models.Model):
      rows = models.ManyToManyField(Row, blank=True)
      name = models.CharField(max_length=255)
  
      def __unicode__(self):
          return self.name
  </code></pre>
  
  <p>and my admin:</p>
  
  <pre><code>class RowInline(admin.TabularInline):
      model = Table.rows.through
      fields = ['name']
  
  
  class TableAdmin(admin.ModelAdmin):
      inlines = [
          RowInline,
      ]
      exclude = ('rows',)
  </code></pre>
  
  <p>However I get this error </p>
  
  <blockquote>
    <p>ImproperlyConfigured at /admin/table_app/table/1/</p>
    
    <p>'RowInline.fields' refers to field 'name' that is missing from the
    form.</p>
  </blockquote>
  
  <p>How is that possible ? </p>