title: Apps aren't loaded yet (when learning signals)
body: <p>I'm trying to learn signals. In the apps.py below there is a couple of commented lines. As soon as I uncomment from .models import ReceiverModel, SenderModel, I get django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.</p>
  
  <p>Could you help me understand what I am doing wrongly?</p>
  
  <p><strong>demo_signals/my_app/models.py</strong></p>
  
  <pre><code>from django.db import models
  from django.dispatch import receiver
  from django.db.models.signals import post_save
  
  class SenderModel(models.Model):
      pass
  
  class ReceiverModel(models.Model):
      pass
  
      @receiver(post_save, sender=SenderModel)
      def my_callback(self, sender, **kwargs):
          print("Saving finished!")
  </code></pre>
  
  <p><strong>demo_signals/demo_signals/settings.py</strong></p>
  
  <pre><code>INSTALLED_APPS = [
      ...
      'my_app.apps.MyAppConfig',
  ]
  </code></pre>
  
  <p><strong>demo_signals/my_app/apps.py</strong></p>
  
  <pre><code>from django.apps import AppConfig
  from django.db.models.signals import Signal
  #from .models import ReceiverModel, SenderModel
  
  
  class MyAppConfig(AppConfig):
      name = 'my_app'
  
      def ready(self):
          print("Here")
          #Signal.connect(self, ReceiverModel, SenderModel)
  </code></pre>
  
  <p><strong>Some part of the traceback</strong></p>
  
  <pre><code>  File "/home/michael/workspace/demo_signals/my_app/apps.py", line 3, in &lt;module&gt;
      from .models import ReceiverModel, SenderModel
    File "/home/michael/workspace/demo_signals/my_app/models.py", line 5, in &lt;module&gt;
      class SenderModel(models.Model):
    File "/home/michael/workspace/PhotoArchive/venv/lib/python3.5/site-packages/django/db/models/base.py", line 94, in __new__
      app_config = apps.get_containing_app_config(module)
    File "/home/michael/workspace/PhotoArchive/venv/lib/python3.5/site-packages/django/apps/registry.py", line 240, in get_containing_app_config
      self.check_apps_ready()
    File "/home/michael/workspace/PhotoArchive/venv/lib/python3.5/site-packages/django/apps/registry.py", line 125, in check_apps_ready
      raise AppRegistryNotReady("Apps aren't loaded yet.")
  </code></pre>