diff --git a/django_etesync/models.py b/django_etesync/models.py
index 7315ee8..c6ec571 100644
--- a/django_etesync/models.py
+++ b/django_etesync/models.py
@@ -44,6 +44,15 @@ class Collection(models.Model):
     def content(self):
         return self.main_item.content
 
+    @cached_property
+    def stoken(self):
+        last_revision = CollectionItemRevision.objects.filter(item__collection=self).last()
+        if last_revision is None:
+            # FIXME: what is the etag for None? Though if we use the revision for collection it should be shared anyway.
+            return None
+
+        return last_revision.uid
+
 
 class CollectionItem(models.Model):
     uid = models.CharField(db_index=True, blank=False, null=True,
diff --git a/django_etesync/serializers.py b/django_etesync/serializers.py
index 03dd0bb..eb50d76 100644
--- a/django_etesync/serializers.py
+++ b/django_etesync/serializers.py
@@ -149,7 +149,7 @@ class CollectionItemSerializer(serializers.ModelSerializer):
 class CollectionSerializer(serializers.ModelSerializer):
     encryptionKey = CollectionEncryptionKeyField()
     accessLevel = serializers.SerializerMethodField('get_access_level_from_context')
-    stoken = serializers.SerializerMethodField('get_stoken')
+    stoken = serializers.CharField(read_only=True)
     content = CollectionItemRevisionSerializer(many=False)
 
     class Meta:
@@ -162,14 +162,6 @@ class CollectionSerializer(serializers.ModelSerializer):
             return obj.members.get(user=request.user).accessLevel
         return None
 
-    def get_stoken(self, obj):
-        last_revision = models.CollectionItemRevision.objects.filter(item__collection=obj).last()
-        if last_revision is None:
-            # FIXME: what is the etag for None? Though if we use the revision for collection it should be shared anyway.
-            return None
-
-        return last_revision.uid
-
     def create(self, validated_data):
         """Function that's called when this serializer creates an item"""
         revision_data = validated_data.pop('content')