The MongoDB Cookbook
> Voting with Atomic Operators

Voting with Atomic Operators

Credit: Kyle Banker

Problem

You want to give your users the ability to vote on things. Whether it's articles, comments, photos, or tweets, it seems like everything needs voteability.

Make sure that each user gets just one vote.
Keep a counter cache on the number of votes.

Solution

The solution is provided in JavaScript; translating to the language of your choice should be pretty straightforward.

1. Store the vote information in the object itself.

Let's say you're building a social news site like Digg. You want your users to be able to vote on submitted stories. Here's a sample story document with all the information required for voting:

{'_id': ObjectId("4bcc9e697e020f2d44471d27"),
 title: 'Aliens discovered on Mars!',
 description: 'Martian'
 vote_count: 0,
 voters: []
}

Notice that we've reserved two fields for voting: the first is an integer caching the number of votes, and the second is a list of voters.

2. Use an atomic update operation for adding and removing votes.

Here you get to see what's great about atomic operators. You can reliably add the vote, without risking a duplicate, in a single operation. Here's the code to update the story above:

// Get the user id who's voting
user_id = ObjectId("4bcc9e697e020f2d44471a15");

// This query succeeds only if the voters array doesn't contain the user
query   = {_id: ObjectId("4bcc9e697e020f2d44471d27"), voters: {'$ne': user_id});

// Update to add the user to the array and increment the number of votes.
update  = {'$push': {'voters': user_id}, '$inc': {vote_count: 1}}

db.stories.update(query, update);

3. If you want to allow users to retract their votes, the code is quite simiar:

The only difference is that we use the $pull operator, and we decrement by passing -1 to $inc.

// This query succeeds when the voter has already voted on the story.
query   = {_id: ObjectId("4bcc9e697e020f2d44471d27"), voters: user_id};

// Update to remove the user from the array and decrement the number of votes.
update  = {'$pull': {'voters': user_id}, '$inc': {vote_count: -1}}

db.stories.update(query, update);

Discussion

One thing to note is that because the operation of step 2 uses the $ne operator, that part of the query can't use an index. This may become a problem if you expect many hundreds of votes per story; any fewer shouldn't be a concern.

By contrast, the query in step 3 can use a compound index efficiently:

db.stories.ensureIndex({'_id': 1, voters: 1});

However, you'd create this index only if you expect people to be changing their votes often (which usually isn't the case).