title: Django model operations with foreign key and double underscore
body: <p>I'm new to Django and just being curious to know more. 
  Assuming I have two models defined as follows:</p>
  
  <pre><code>class ModelA(models.Model):
      Item1 = models.Charfield()
      description = models.Charfield()
  
  class ModelB(models.Model):
      ItemTypes = models.CharField(ModelA, on_delete=models.CASCADE)
  </code></pre>
  
  <p>When trying to reference or use the model, you use them like this (still assuming):</p>
  
  <pre><code>something = ModelB.objects.filter(ItemType__Item)
  </code></pre>
  
  <p>My question is on the double underscore <code>__</code>.</p>
  
  <p>I'm just curious to know:</p>
  
  <ol>
  <li><p>What does the code at the back end look like.</p></li>
  <li><p>Is there a valid syntax with a triple underscore or a single underscore?</p></li>
  </ol>