[{"_id":ObjectId("5a934e000102030405000000"),"letters":["a","b","c","d"]},{"_id":ObjectId("5a934e000102030405000001"),"letters":["a","b"]},{"_id":ObjectId("5a934e000102030405000002"),"letters":["a"]},{"_id":ObjectId("5a934e000102030405000003"),"letters":["x","a","b"]}]

db.collection.find({"letters.0":"a","letters.1":"b"})