Try ordering via @property field order_by

webapp/models.py

@@ -106,7 +106,7 @@ class SignupForm(models.Model):
 
     @property
     def signups(self):
-        return Signup.objects.filter(signupForm=self)
+        return Signup.objects.filter(signupForm=self).order_by('pk')
 
     class Meta:
         verbose_name = _('Signup form')

webapp/serializers.py

@@ -46,16 +46,16 @@ class SignupSerializer(serializers.ModelSerializer):
 
 class SignupFormSerializer(serializers.HyperlinkedModelSerializer):
     questions = serializers.JSONField()
-    signups = serializers.SerializerMethodField()
+    signups = serializers.SlugRelatedField(
+        slug_field='list_name',
+        read_only=True,
+        required=False,
+    )
 
     class Meta:
         model = SignupForm
         fields = ('id', 'title', 'visible', 'start_time', 'end_time', 'questions', 'schema', 'signups', 'quota')
 
-    def get_signups(self, instance):
-        names = instance.list_name.order_by('pk')
-        return names
-
 
 class EventSerializer(serializers.ModelSerializer):
     signupForm = SignupFormSerializer(