title: deleting related models django
body: <p>i currently have this models how do i delete the logo without deleting the company model.Please sample codes if possible thanks.</p>
  
  <pre><code>class Picture(models.Model):
      owner = models.ForeignKey(User,blank = True)
      caption = models.CharField(max_length=150, blank=True, null=True)
      image = ImageField(upload_to='images/',blank = True, null = True)
  
  class Company(GenericUser):
      company_name = models.CharField(max_length=150,blank = True,null = True)
      logo = models.ForeignKey(Picture,blank = True,null = True)
  </code></pre>
  
  <p>this is a model how will i then remove the foto from the land model the model looks like this.</p>
  
  <pre><code>class Land(Properies):
      photo = models.ManyToManyField(Picture,blank=True,related_name='Land_Pictures',null = True)
  </code></pre>
  
  <p>i tried this bit it dose not work</p>
  
  <pre><code>checked_list = []
  start = 1            
  land_photos = sorted(list(land.photo.select_related()),reverse =True)
  while start &lt; 8:
      photo = 'photo%s' % start
      checked = form.cleaned_data[photo]
      if checked != None:
          checked_list.append(land_photos[start - 1])
          start += 1            
  for a_foto in checked_list:
  land.photo.remove(a_foto)
  try:
      a_foto.remove_all_file()
      a_foto.delete()
  except OSError:
      pass
  </code></pre>
  
  <p>i keep getting an error like that id is set to none and if i hit refresh it works i think</p>
  
  <pre><code>Exception Type:     AssertionError
  Exception Value:    
  Picture object can't be deleted because its id attribute is set to None.
  </code></pre>