title: Check if record exists before saving
body: <p>I have a Customer class linked to several other classes via foreign key.  I want to have the forms working in such a way that if the submitted customer record already exists, then this record will be used as the foreign key for the dependent class.  </p>
  
  <p>I've declared the Customer class as:</p>
  
  <pre><code>class Customer(CustomerBaseInfo):
     date_time_added = models.DateTimeField(default=datetime.today)
     source = models.ForeignKey(Source, blank=False)
     email_address = models.CharField(max_length=75)
     phone_number = models.CharField(max_length=20)
     preffered_contact_method = models.ForeignKey(PreferredContact)
     best_time_to_contact = models.ForeignKey(BestTime)
     def __unicode__(self):
       return self.first_name
  </code></pre>
  
  <p>One of the classes that link to the Customer class look like this:</p>
  
  <pre><code>class Message(models.Model):
     date_time_added = models.DateTimeField(default=datetime.today)
     message_type = models.ForeignKey(MessageType)
     customer = models.ForeignKey(Customer)
     representative = models.ForeignKey(Representative)
     vehicle = models.CharField(max_length=80)
     message = models.TextField(null=True)
     def __unicode__(self):
       return self.date_time_added
  </code></pre>