[{"_id":ObjectId("5a934e000102030405000000"),"my_letter":"d"},{"_id":ObjectId("5a934e000102030405000001"),"my_letter":"a"},{"_id":ObjectId("5a934e000102030405000002"),"my_letter":"b"}]

db.collection.aggregate([{$match:{my_letter:{$in:["a","b"]}}}])