title: Return a model field in django view set
body: <p>I have a problem with my views,I'm trying to write a view to return my user membership if user has in site,but I get an error
  membership model is:</p>
  <pre><code>
      class Membership(models.Model):
      membership_type = models.CharField(
          choices=MEMBERSHIP_CHOICES,
          default='Free',
          max_length=30)
      price = models.DecimalField(max_digits=10, default=0, decimal_places=3)
  
      def __str__(self):
          return self.membership_type
  
  </code></pre>
  <p>user membership is:</p>
  <pre><code>
      class UserMembership(models.Model):
      user = models.ForeignKey('accounts.User', on_delete=models.CASCADE, related_name='user_membership')
      membership = models.ForeignKey(Membership, on_delete=models.DO_NOTHING, null=True, related_name='membership',
                                     default='Free')
  
      def __str__(self):
          return self.user.email
  
  </code></pre>
  <p>view is:</p>
  <pre><code>
      class MembershipView(viewsets.ModelViewSet):
      model = UserMembership
      serializer_class = UserMemberShipSerializer
  
      def get_queryset(self):
          user_membership_qs = UserMembership.objects.get(user=self.request.user)
          if user_membership_qs:
              membership = user_membership_qs.membership
              return membership
  
  </code></pre>
  <p>I tried Return Response(membership) too,but didn't work,</p>
  <p>Error is: object of type 'Membership' has no len()</p>