title: Django, auto setting a field during a save, based on other Admin page inputs
body: <p>I'm looking for the correct way to set full_name in SuperPerson instance.</p>
  
  <pre><code>class Suffix(models.Mode):
      suffix = models.CharField(max_length=255)
      def __unicode__(self):
          return u'%s'%(self.suffix)
  
  class Person(models.Model):
      first_name= models.CharField(max_length=255)
      last_name= models.CharField(max_length=255)
      suffixes= models.ManyToManyField(Suffix, blank=True, null=True)
      full_name= models.CharField(max_length=255)
  
  class SuperPerson(Person):
      ignore_this_field= model.CharField(max_length=255)
  </code></pre>
  
  <p>full_name is hidden from the user on the Admin page, and is to be automatically be updated based on the other inputs on Admin page when the Admin page save button is hit.</p>
  
  <p>I have tried overriding save like this and variations:</p>
  
  <pre><code>def save(self, *args, **kwargs):
      # Attempt to get data into the database so I can access it
      super(SuperPerson,self).save(*args,**kwargs)
  
      self.full_name = self.first_name + self.last_name 
      for suf in self.suffixes.all():
          self.full_name+= suf.__unicode__()
  
      # Now save the copy with full_name set as I wish
      super(SuperPerson,self).save(*args,**kwargs)
  </code></pre>
  
  <p>This method works if I hit the save button in the Admin page twice, which is unacceptable for my use cases, seems like the new self.suffixes I have entered from the Admin page hasn't made it into database with the first super.save when I call self.suffixes.all().</p>
  
  <p>I tried making full_name a property with decorator, but I also need to be able to filter Person and SuperPerson dbs using full_name, so that didn't work, unless someone can tell me how to filter with a property. Though I would rather have the value saved in the db.</p>
  
  <p>I tried pre_save and post_save signals  - neither worked.</p>
  
  <pre><code>@receiver(pre_save, sender=SuperPerson)
  def set_full_name(sender, instance, **kwargs):
      instance.full_name = instance.first_name + instance.last_name
  
      for suf in instance.suffixes.all():
          instance.full_name+= ', ' + suf.__unicode__()
  </code></pre>
  
  <p>Edit: - this has same effect - the instance suffixes do not match what was in the Admin page.</p>
  
  <p>What is the right way to save full_name based on other inputs?  Oh and I'm hoping to avoid messing with Admin forms.</p>
  
  <p>ADDITIONAL INFORMATION:
  It seems the problem is specifically that the suffixes field is not being updated by the time I'm trying to use it.  I can update full_name to something else, like appending a string representing the current date, I just cannot access the suffixes.</p>
  
  <p>Thanks, Dale</p>
  
  <p>SOLUTION:</p>
  
  <pre><code>@receiver(m2m_changed, sender=Person.suffixes.through)
  def set_full_name_after_ManyToMany_saved(sender, instance, **kwargs):
      instance.full_name = instance.first_name + instance.last_name
      for suf in instance.suffixes.all():
          instance.full_name+= ', ' + suf.__unicode__()
      print 'Saving As', instance.full_name
      instance.save()
  </code></pre>
  
  <p>I'm curious why I had to use Person.suffixes.through instead of SuperPerson, Suffixes or Person.suffixes - is there good documentation on this somewhere, I couldn't find it.  And, it runs the code 4 times, but at least ends up with the correct result in the final run.</p>
  
  <p>Many thanks to Danny and burhan</p>