Try SerializerMethodField for ordering list_names

webapp/serializers.py

@@ -46,17 +46,15 @@ class SignupSerializer(serializers.ModelSerializer):
 
 class SignupFormSerializer(serializers.HyperlinkedModelSerializer):
     questions = serializers.JSONField()
-    signups = serializers.SlugRelatedField(
-        slug_field="list_name",
-        many=True,
-        read_only=True,
-        required=False
-    )
+    signups = serializers.SerializerMethodField()
 
     class Meta:
         model = SignupForm
         fields = ('id', 'title', 'visible', 'start_time', 'end_time', 'questions', 'schema', 'signups', 'quota')
-        ordering = ['signups', 'pk']
+
+    def get_signups(self, instance):
+        names = instance.list_name.order_by('pk')
+        return names
 
 
 class EventSerializer(serializers.ModelSerializer):