How to form an unordered key with many elements in mongodb

Question

I'm attempting to use mongodb to implement a simple messaging system between two users in mongo. I want to be able to take two users, user0 and user1, and search for their entry in a collection. If the entry for those two users doesn't exist I want to create it and then add the message that was sent to its message field. If it does exist I just want to push the message to the message field.

I'm not really sure the best way to implement this.

db.privateChat.update(
    {between:{$all:['user0', 'user1']}}, 
    {$push:{message:'text'}}, {upsert:true}
)

And other similar entry schemes but they don't work. They produce the error:

"Cannot create base during insert of update. Caused by :ConflictingUpdateOperators Cannot update 'between' and 'between' at the same time"

I can think of other ways to do this producing a symmetric key (where the order of the users don't matter for the purposes of the search) from say adding the hashes together or a query that checks if either messenger0 or messenger1 is either user0 or user1 but these don't seem like great ways of doing it. Is this totally the wrong approach?

Thanks.

Answer 1

I think this could be solved by design. let say that we have document in collection chats;

chat{
_id,
between[arrayOfIds],
startTime,
events[
{message:{
      fromUserId,
      timeStamp,
      data}
}}
]}
}

then messages will be stored in message object inside chat .

App will be aware of chat _id so there will be no issues when you will have a group chat between more than 2 users.

This approach will allow you to prevent overflowing document size limitation as you could start new chat entry every week, day, etc...

Have a fun!