title: Foreign key field disappears in swagger docs after adding depth attribute in Serializer
body: <p>Whenever I define the <code>depth</code> attribute, the foreign key field from swagger docs in POST section disappears. That seems strange because I required <code>depth = 1</code> when I want related data in my <code>GET</code> request. So I can not remove this in order to get this related field parameter in the <code>POST</code> section. </p>
  
  <p><em>Here is the case.</em></p>
  
  <p><strong>Model:</strong></p>
  
  <pre><code>from django.db import models
  from django.conf import settings
  # Create your models here.
  
  User = settings.AUTH_USER_MODEL
  
  class Todo(models.Model):
      user = models.ForeignKey(User)
      title = models.CharField("Title", max_length=255)
      completed = models.BooleanField("Completed")
  </code></pre>
  
  <p><strong>Serializer</strong> without <code>depth =1</code>.</p>
  
  <pre><code>from rest_framework import serializers
  from models import Todo
  
  class TodoSerializer(serializers.HyperlinkedModelSerializer):
     class Meta:
         model = Todo
  </code></pre>
  
  <p><strong>Swagger output:</strong>
  <a href="https://i.stack.imgur.com/Feuu9.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Feuu9.png" alt="enter image description here"></a></p>
  
  <p>Now If I add <code>depth = 1</code> than <strong>Swagger does not display related field</strong>.
  <a href="https://i.stack.imgur.com/0Xn9z.png" rel="noreferrer"><img src="https://i.stack.imgur.com/0Xn9z.png" alt="enter image description here"></a></p>
  
  <p>Let me know if anyone has any clue about this.</p>
  
  <p>Thanks :)</p>