title: Is it possible to embed an ArrayModelField inside ArrayModelField?
body: <p>I'm working on a project using django connecting to a MongoDB database using djongo. I have a few models and a few abstract models that I embed as ArrayModelFields. However when I try to use an ArrayModelField inside a model that is in-turn referenced as an ArrayModelField I get a 'nonetype' object is not iterable (not sure why). </p>
  
  <p>I've gotten the system to work by combining the two models (Grade &amp; Detail) into one in this case (after realizing the Grade model didn't need to exist) but is it possible at all to do what I wanted to do initially?</p>
  
  <p>Before I've tried various embedding options for lists and arrays as listed in the djongo documentation but nothing else works.</p>
  
  <pre><code>class Grade(models.Model):
      kihon_Grade = models.CharField(max_length = 2, null = True)
      kata_Grade = models.CharField(max_length = 2, null = True)
      kumite_Grade = models.CharField(max_length = 2, null = True)
      class Meta:
          abstract = True
  
  class Detail(models.Model):
      member = models.ForeignKey(to = Member, on_delete = None)
      rank = models.CharField(max_length = 10, null = True)
      grade = models.ArrayModelField(model_container = Grade, null = True)
      result = models.CharField(max_length = 10, null = True)
      class Meta:
          abstract = True
  
  class Grading(models.Model):
      _id = models.ObjectIdField()
      examiner = models.CharField(max_length = 25, null = True)
      date = models.DateField(null = True, blank = True)
      detail = models.ArrayModelField(model_container=Detail, null=True, blank=True)
      def __str__(self):
          return self.date
  </code></pre>
  
  <p>I get the following exception thrown.</p>
  
  <blockquote>
    <p>Django Version:   2.0</p>
    
    <p>Exception Type:   TypeError</p>
    
    <p>Exception Value:  'NoneType' object is not iterable</p>
    
    <p>Exception Location:
      D:\COMP405\COMP405\env\lib\site-packages\djongo\models\fields.py in
    has_changed, line 329</p>
    
    <p>Python Executable:    D:\COMP405\COMP405\env\Scripts\python.exe</p>
    
    <p>Python Version:   3.7.2</p>
  </blockquote>