Sort by array length

Question

How can I sort a query by the number of elements of an array field?

Let say I have records like

{
 title: '',
 author: '',
 votes: [id,id,id]
}

I would like to do sort by the length of the array votes

Answer 1

Use the aggregation framework with help from the $size operator from MongoDB 2.6 and upwards:

db.collection.aggregate([
    // Project with an array length
    { "$project": {
        "title": 1,
        "author": 1,
        "votes": 1,
        "length": { "$size": "$votes" }
    }},

    // Sort on the "length"
    { "$sort": { "length": -1 } },

    // Project if you really want
    { "$project": {
        "title": 1,
        "author": 1,
        "votes": 1,
    }}
])

Simple enough.

If you do not have a version 2.6 available you can still do this with a little more work:

db.collection.aggregate([
    // unwind the array
    { "$unwind": "$votes" },

    // Group back
    { "$group": {
        "_id": "$id",
        "title": { "$first": "$title" },
        "author": { "$first": "$author" },
        "votes": { "$push": "$votes" },
        "length": { "$sum": 1 }
    }},

    // Sort again
    { "$sort": { "length": -1 } },

    // Project if you want to
    { "$project": {
        "title": 1,
        "author": 1,
        "votes": 1,
    }}
])

That is pretty much it.