Database
[{lastMessageAt:ISODate("2016-05-18T18:00:00Z"),participants:[{user:"12345",lastReadAt:ISODate("2016-05-18T16:00:00Z")},{user:"54321",lastReadAt:ISODate("2016-05-18T21:00:00Z")}]}]
Query
db.collection.aggregate([{$match:{"participants.user":"12345",$expr:{$lt:["$participants.lastReadAt","$lastMessageAt"]}}}])