{
"_id" : ObjectId("5a5f814487c320156094c144"),
"sender_id" : "123",
"iso_number" : "ABC-DEF-123",
"subject" : "Sample Memo",
"content" : "This is a sample memorandum sent through postman.",
"recipients" : [
    {
        "faculty_number" : 222,
        "_id" : ObjectId("5a5f814487c320156094c146"),
        "status" : "Sent"
    },
    {
        "faculty_number" : 111,
        "_id" : ObjectId("5a5f814487c320156094c145"),
        "status" : "Sent"
    }
],
"memo_created" : ISODate("2018-01-17T17:00:52.104Z"),
"__v" : 0
}

db.getCollection('memos').update({"_id": id, "recipients.faculty_number": faculty_number, "recipients.$.memo_seen": {$exists: false}}, {$set: {"recipients.$.memo_seen": timestamp}})