title: Nested Serialization using SlugRelatedField
body: <p>I have a model structure as such:</p>
  
  <pre><code>class Student(models.Model):
      created = models.DateTimeField(editable=False)
      # should modifieid be editable?
      modified = models.DateTimeField(editable=False)
      english_name = models.CharField(max_length=SHORT_LENGTH)
      # TODO: make sure chinese characters work!
      chinese_name = models.CharField(max_length=SHORT_LENGTH)
      # student_id = ####
  
      def save(self, *args, **kwargs):
          # store curr time ahead so we don't account for processor cycles
          curr_time = timezone.now()
  
          # if we don't have an ID, we're brand new!
          if not self.id:
              self.created = curr_time
          self.modified = curr_time
          return super(Student, self).save(*args, **kwargs)
  
  class SeminarMembership(models.Model):
      student = models.ForeignKey('Student')
      seminar = models.ForeignKey('Seminar', related_name = "memberships")
  
  class Seminar(models.Model):
      created = models.DateTimeField(editable=False)
      modified = models.DateTimeField()
      #userID = ############
      title = models.CharField(max_length=LONG_LENGTH)
      #description
  
      def save(self, *args, **kwargs):
          # store curr time ahead so we don't account for processor cycles
          curr_time = timezone.now()
  
          # if we don't have an ID, we're brand new!
          if not self.id:
              self.created = curr_time
          self.modified = curr_time
          return super(Seminar, self).save(*args, **kwargs)
  </code></pre>
  
  <p>Where SeminarMembership is a pivot that establishes Student-Seminar Pairings.</p>
  
  <p>I'm attempting to have DRF serialize an individual Seminar entity such that the JSON response includes all of the students that are registered with that seminar through the pivot. To do this, I retrieve all of the memberships associated with a certain Seminar (many=True):</p>
  
  <pre><code>class SeminarSerializer(serializers.ModelSerializer):
      memberships = SeminarMembershipSerializer(many=True)
      class Meta:
          model = Seminar
          read_only_fields = ('title',)
  </code></pre>
  
  <p>Then, since the Serializer for SeminarMembership is as follows:</p>
  
  <pre><code>class SeminarMembershipSerializer(serializers.ModelSerializer):
      # needs to be read_only?
      student = StudentSerializer(many=False)
      seminar = serializers.PrimaryKeyRelatedField(many=False, read_only=True)
  
      class Meta:
          model = SeminarMembership
  </code></pre>
  
  <p>When I modify SeminarSerializer to be the following, I get a "Student object not serializable" error:</p>
  
  <pre><code>   class SeminarSerializer(serializers.ModelSerializer):
          memberships = serializers.SlugRelatedField(many=True, read_only=True, slug_field='student')
          class Meta:
              model = Seminar
              read_only_fields = ('title',)
  </code></pre>
  
  <p>Theoretically, DRF should recognize that the SlugRelatedField should be serialized using StudentSerializer from the implementation of SeminarMembershipSerializer, and then proceed serialize the Foreign Student Object field for use in the JSON response. But then I run into a "Student object" not JSON serializable.</p>