title: Extract nested value from a Django jsonfield
body: <p>Is there a way to query an object, 'extract' a nested piece of data from a <code>JSONField</code> field and then make it available as a custom, temporary field on each instance of the Queryset?</p>
  
  <p>In my use case, I'm storing overflow metadata from Twitter's API in a <code>data</code> field for later use. I'd like to be able to access the nested field <code>followers_count</code> within <code>TwitterPost.data</code>.</p>
  
  <p>I've read the docs about how to filter based on nested values but not how to extract it as a temporary field when generating a queryset.</p>
  
  <p>Similarly, I've read the <code>annotate</code> docs for ways to create a custom temporary field but the examples all use aggregation functions on simple fields, so not <code>JSONFields</code>.</p>
  
  <p>Thanks in advance for any suggestions.</p>
  
  <p>Example model:</p>
  
  <pre><code>from django.contrib.postgres.fields import JSONField
  
  class TwitterPost(models.Model):
      id = models.IntegerField()
      data = JSONField()
  </code></pre>
  
  <p>Example JSON value for the <code>data</code> field:</p>
  
  <pre><code>{
    'followers_count': 7172,
    "default_profile_image": false,
    "profile_text_color": "000000"
  }
  </code></pre>
  
  <p>Pseudocode for what I'd like to be able to do:</p>
  
  <pre><code>TwitterPost.objects.annotate(followers_count=instance.data.followers_count)
  </code></pre>