Using functions for creating an index in MongoDB? - mongodb

Question

I have a blog that allow users to vote for articles. The documents of an article looks like:
{
title: "title of post",
body: "text text text text text text",
votes: [
{user: "user1", points: 2},
{user: "user13", points: 1},
{user: "user30", points: 1},
{user: "user2", points: -1},
{user: "user51", points: 3},
],
sum_of_votes: 6
}
I'd like to sort by the number of votes the post received. At the moment I added a field sum_of_votes that needs to be updated every time somebody voted. Since votes already contains the raw-data for sum_of_votes I thought there might be a more elegant way. I came up with two ideas:
Using a function while creating an index, for example:
db.coll.ensureIndex({{$sum: votes.points}: 1})
Having a dynamic field. The document could look like this:
{
title: "title of post",
body: "text text text text text text",
votes: [
{user: "user1", points: 2},
{user: "user13", points: 1},
{user: "user30", points: 1},
{user: "user2", points: -1},
{user: "user51", points: 3},
],
sum_of_votes: {$sum: votes.points}
}
In those cases I only had to update the votes-array. Is something like this possible in MongoDB?

Answer 1

Depending on how you do the update on the votes array, either as updating the document by inserting a new vote in the votes array or updating an existing vote, an approach you could take is use the $inc operator on the sum_of_votes with the value you are using to update the votes.points with. For example, in the case where you are inserting a new vote for a user with value in variable userId and the actual vote points in variable points (which must be a number), you could try:
db.articles.update(
{"_id" : article_id},
{
"$addToSet": {
"votes": {
"user": userId,
"points" : points
}
},
"$inc": { "sum_of_votes": points }
}
)
And you will need to factor in Neil Lunn's point where you wouldn't want the same user to vote more than once in your updates.