Question

I’m trying to make a MongoDB database for a simple voting system. If I were to draw a schema, then it looks something like this:

User {
    name: String
  , email: String
}

Vote {
    message: String
  , voters: [ObjectId(User._id)]
}

I have some questions about this design when there are a lot of voters for one vote:

1. Sending the whole voters array to the client’s side (not to mention caching it in memory) is very expensive, right? Is there a way to get the Vote in a “shallow” way, so when I need vote.voters, it will make another database request to the array of voters?
2. If a voter has voted already, I want to check that and not count his vote. To do that, is there a query I can run in the array of embedded voters to quickly find this?
3. When showing votes, I’d want to show the number of votes without fetching the voters array to the client side. Is there some kind of count query I can run to count the voters length?

Answer 1

I would add a bit of redundancy to the schema to avoid some of the potential problems you mention. I assume you want to 1) quickly count the number of votes and 2) make sure a user cannot vote twice.

One way to achieve this is to keep both a list of users and a count of votes, and add a clause to the update query that makes sure that a vote is only added if the user’s ID is not in the list of voters:

var query = {_id: xyz, voters: {$ne: user_id}
var update = {$inc: {votes: 1}, $push: {voters: user_id}}
db.votes.update(query, update, true)

(the last parameter is upsert, a very nice feature of Mongo)

Then, if you want to show the number of votes you can limit the properties of the result to the votes property:

db.votes.find({_id: xyz}, {votes: true})

You can find a complete description of more or less exactly what you want to do here: http://cookbook.mongodb.org/patterns/votes/