Add commentary on ttl index

services/notifications/README.md

@@ -2,3 +2,13 @@ notifications-sharelatex
 ===============
 
 An API for managing user notifications in ShareLaTeX
+
+
+database indexes
+================
+
+For notification expiry to work, a ttl index on `notifications.expiresFrom` must be created:
+
+```javascript
+db.notifications.createIndex({expiresFrom: 1}, {expireAfterSeconds: (60*60*24*30)})
+```

services/notifications/app/coffee/Notifications.coffee

@@ -29,6 +29,8 @@ module.exports =
 					templateKey: notification.templateKey
 					expires: notification.expires || false
 				# ttl index on `expiresFrom` field
+				# in Mongo, TTL indexes only work on date fields,
+				# and ignore the document when that field is missing
 				if doc.expires
 					doc.expiresFrom = new Date()
 				db.notifications.insert(doc, callback)