title: Django 1.7 upgrade error: AppRegistryNotReady using serializers from rest_framework
body: <p>I get this traceback:</p>
  
  <pre><code>Traceback (most recent call last):
    File "./manage.py", line 38, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
      utility.execute()
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/core/management/__init__.py", line 354, in execute
      django.setup()
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/__init__.py", line 21, in setup
      apps.populate(settings.INSTALLED_APPS)
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
      app_config.import_models(all_models)
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/apps/config.py", line 197, in import_models
      self.models_module = import_module(models_module_name)
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.py", line 37, in import_module
      __import__(name)
    File "/Users/mgregory/Documents/cm_central/cmh_server/models.py", line 88, in &lt;module&gt;
      class VersionSerializer(serializers.ModelSerializer):
    File "/Users/mgregory/Documents/cm_central/cmh_server/models.py", line 89, in VersionSerializer
      brzs= BrzSerializer(many=True)
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/rest_framework/serializers.py", line 198, in __init__
      self.fields = self.get_fields()
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/rest_framework/serializers.py", line 234, in get_fields
      default_fields = self.get_default_fields()
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/rest_framework/serializers.py", line 732, in get_default_fields
      reverse_rels = opts.get_all_related_objects()
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/db/models/options.py", line 498, in get_all_related_objects
      include_proxy_eq=include_proxy_eq)]
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/db/models/options.py", line 510, in get_all_related_objects_with_model
      self._fill_related_objects_cache()
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/db/models/options.py", line 533, in _fill_related_objects_cache
      for klass in self.apps.get_models(include_auto_created=True):
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/utils/lru_cache.py", line 101, in wrapper
      result = user_function(*args, **kwds)
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/apps/registry.py", line 168, in get_models
      self.check_models_ready()
    File "/Users/mgregory/Documents/virtualenvs/cm_central/lib/python2.7/site-packages/django/apps/registry.py", line 131, in check_models_ready
      raise AppRegistryNotReady("Models aren't loaded yet.")
  django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.
  </code></pre>
  
  <p>from this code (models.py):</p>
  
  <pre><code># Serializers for transmitting CMx install information over HTTP
  
  class BrzSerializer(serializers.ModelSerializer):
      class Meta:
          model = Brz
          fields=('filename',)
  
  class VersionSerializer(serializers.ModelSerializer):
      brzs= BrzSerializer(many=True)
      class Meta:
          model = Version
          fields=('name', 'for_mac', 'for_windows', 'brzs')
  </code></pre>
  
  <p>It sounds like it's telling me "your VersionSerializer can't have a BrzSerializer, because you haven't registered that yet".   </p>
  
  <p>I've looked at other SO questions relating to AppRegisteryNotReady, but didn't find one that matches this symptom.   Surely I have to be able to define a chain of dependent models like this?</p>