title: Django create_or_update(), unique_together, unique, and primary
body: <p>I'm still trying to fully understand Django models, let's say I have the following model:</p>
  
  <pre><code>class Manufacturer(models.Model):
      name = models.CharField(amx_length=255, unique=True)
  
  class Car(models.Model):
      name = models.CharField(max_length=255)
      color = models.CharField(max_length=255)
      manufacturer = models.ForeignKey(Manufacturer)
      unique_together = ('manufacturer', 'name')
  </code></pre>
  
  <p>And I run the following commands:</p>
  
  <pre><code>honda = Manufacturer.objects.get(name='Honda') #Let's assume this object already exists
  car = Car.objects.get_or_create(name='Honda Accord', color='Red', manufacturer=honda)
  car = Car.objects.get_or_create(name='Honda Accord', color='Green', manufacturer=honda)
  </code></pre>
  
  <p>Will Django simply update the existing car to change the color from Red to Green? Or will issues arise because I don't have a <code>primary_key</code> set?</p>
  
  <p>Also, does it matter if I use <code>primary_key</code> or <code>unique</code> or are they the same? I'm using a mySQL backend if it matters.</p>