Modify user serializer update to have expected behavior

Updates should add to many to many relationships not set them.

geeksbot_web/users/serializers.py

@@ -1,4 +1,6 @@
 from rest_framework import serializers
+from rest_framework.serializers import raise_errors_on_nested_writes
+from rest_framework.utils import model_meta
 
 from users.models import User
 from users.models import UserLog
@@ -45,7 +47,37 @@ class UserSerializer(serializers.HyperlinkedModelSerializer):
             prev_discrims = set(instance.previous_discriminators or [])
             prev_discrims.add(instance.discriminator)
             validated_data['previous_discriminators'] = list(prev_discrims)
-        return super(UserSerializer, self).update(instance, validated_data)
+        if 'guild' in validated_data and 'guilds' not in validated_data:
+            validated_data['guilds'] = [validated_data['guild']]
+
+        # Copied with slight changes from super.update()
+        raise_errors_on_nested_writes('update', self, validated_data)
+        info = model_meta.get_field_info(instance)
+
+        # Simply set each attribute on the instance, and then save it.
+        # Note that unlike `.create()` we don't need to treat many-to-many
+        # relationships as being a special case. During updates we already
+        # have an instance pk for the relationships to be associated with.
+        m2m_fields = []
+        for attr, value in validated_data.items():
+            if attr in info.relations and info.relations[attr].to_many:
+                m2m_fields.append((attr, value))
+            else:
+                setattr(instance, attr, value)
+
+        instance.save()
+
+        # Note that many-to-many fields are set after updating instance.
+        # Setting m2m fields triggers signals which could potentially change
+        # updated instance and we do not want it to collide with .update()
+        for attr, value in m2m_fields:
+            field = getattr(instance, attr)
+            if isinstance(value, (list, tuple, set)):
+                field.add(*value)
+            else:
+                field.add(value)
+
+        return instance
 
 
 class UserLogSerializer(serializers.ModelSerializer):