title: Lack of data with serialization model with ManyToManyField
body: <p>Here's are examples I have:</p>
  
  <p><strong>models.py:</strong></p>
  
  <pre><code>class Example(models.Model):
      title = models.CharField(...)
      description = models.CharField(...)
  
  class Foo(models.Model):
      example = models.ManyToManyField(Example)
  </code></pre>
  
  <p><strong>serializers.py:</strong></p>
  
  <pre><code>class FooSerializer(serializers.ModelSerializer):
      class Meta:
          model = Foo
          fields = '__all__'
          depth = 1
  </code></pre>
  
  <p><strong>views.py:</strong></p>
  
  <pre><code>...
  serialized_data = [FooSerializer(foo).data for foo in Foo.objects.all().get]
  </code></pre>
  
  <p>In output, I receive only <code>Example</code>'s IDs, but is there any way I could get title and description fields also (details of m2mfield)? As I understand, <code>Foo.objects.all().get</code> simply doesn't contain this data, but maybe I could somehow get it and use it?
  I could also rebuild models if needed, but currently I use m2mf because of needs to contain multiple objects as related to this model data.</p>
  
  <p><strong>update</strong></p>
  
  <p><strong>models.py</strong>:</p>
  
  <pre><code>class Event(models.Model):
      ts = models.BigIntegerField(editable=False)
  
  class Foo(Event):
      user = models.ForeignKey(User, ...)
      example = *...(remains to be the same)*
      foos = models.ForeignKey('self', **somemore** null=True)
  </code></pre>
  
  <p><strong>serializers.py</strong>:</p>
  
  <pre><code>class EventSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = Event
          fields = '__all__'
  
      def to_representation(self, instance):
          result = {'ts': instance.ts}
          if isinstance(instance, Foo):
              result['foo'] = FooSerializer(instance).data
          return result
  
  class UserSerializer(serializers.ModelSerializer):
      class Meta:
          model = User
          fields = ('id', 'username')
  
  class FooSerializer(serializers.ModelSerializer):
     # user = UserSerializer(read_only=True) # with this I have an error: Got AttributeError when attempting to get a value for field 'username' on #serializer 'UserSerializer'
  
      class Meta:
          model = Foo
          fields = '__all__'
          depth = 1
  </code></pre>