diff --git a/aggrec/db_models.py b/aggrec/db_models.py
index 32479c5..794aa8f 100644
--- a/aggrec/db_models.py
+++ b/aggrec/db_models.py
@@ -9,10 +9,7 @@ class AggregateType(Enum):
 
 
 class AggregateMetadata(Document):
-    meta = {
-        "collection": "aggregates",
-        "indexes": [{"fields": ["content_digest"], "unique": True, "sparse": True}],
-    }
+    meta = {"collection": "aggregates"}
 
     creator = StringField()
 
@@ -22,7 +19,7 @@ class AggregateMetadata(Document):
 
     content_type = StringField()
     content_length = IntField()
-    content_digest = StringField()
+    content_digest = StringField(unique=True, sparse=True)
 
     s3_bucket = StringField()
     s3_object_key = StringField()