title: Django Model - cannot serialize into migration files
body: <p>I have 2 models Car and Offer. The car table is an existing table filled with vehicle specifications. In the django admin I want to map an offer to an existing car in the specs table. This means that when a click on an offer in admin, I want to be able to see a list with all cars - find the correct one - and save it on the offer. Ive done this by populating the foreignkey field with a list of choices based on the the existing car objects.</p>
  
  <p>models.py:</p>
  
  <pre><code>class Car(models.Model):
      brand = models.TextField(max_length=300, default= "")
      model = models.TextField(max_length=300, default= "")
      edition = models.TextField(max_length=300, default= "")
      engineVolume = models.FloatField(default=0.0)
  
      def __unicode__(self):
        return smart_unicode(self.brand)
  
  carIds = []
  idx = 1
  for car in Car.objects.all():
          carIds.append((idx,  car))
          idx = idx + 1
  
  
  class Offer(models.Model):
      stringUrl = models.TextField(max_length=300)
      extractionDate = models.DateTimeField(default=datetime.datetime.now, blank=True)
      cars = models.ForeignKey(Car, default= "", choices=carIds, null=True, to_field='id')
  </code></pre>
  
  <p>this works perfectly. I click on an offer in admin and I see a selectionbox populated with all existing cars. I find the correct car, save it, and the offers´s foreignkey car id in the database points to the correct vehicle.
  But suddenly when I want to make later migrations django says it cannot serialze the car object?</p>
  
  <pre><code>Traceback (most recent call last):
  File "manage.py", line 10, in &lt;module&gt;
  execute_from_command_line(sys.argv)
  File "/usr/local/lib/python2.7/site-   packages/django/core/management/__init__.py", line 338, in    execute_from_command_line
  utility.execute()
  File "/usr/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 330, in execute
  self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/base.py", line 390, in run_from_argv
  self.execute(*args, **cmd_options)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/base.py", line 441, in execute
  output = self.handle(*args, **options)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/commands/makemigrations.py", line 143, in handle
  self.write_migration_files(changes)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/commands/makemigrations.py", line 171, in write_migration_files
  migration_string = writer.as_string()
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 166, in as_string
  operation_string, operation_imports = OperationWriter(operation).serialize()
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 124, in serialize
  _write(arg_name, arg_value)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 87, in _write
  arg_string, arg_imports = MigrationWriter.serialize(_arg_value)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 377, in serialize
  return cls.serialize_deconstructed(path, args, kwargs)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 268, in serialize_deconstructed
  arg_string, arg_imports = cls.serialize(arg)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 303, in serialize
  item_string, item_imports = cls.serialize(item)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 303, in serialize
  item_string, item_imports = cls.serialize(item)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 465, in serialize
  "topics/migrations/#migration-serializing" % (value, get_docs_version())
  `enter code here`ValueError: Cannot serialize: &lt;Car: Nissan&gt;
  There are some values Django cannot serialize into migration files.
  For more, see     https://docs.djangoproject.com/en/1.8/topics/migrations/#migration-    serializing
  </code></pre>
  
  <p>I have no idea why this error occurs. I' m new to django and the just starting with the admin. I´ve been reading up on serialzation and deconstruction but cant see how to apply it here? Perhaps I should follow a different route to achieve what I want?</p>