Question

I need to sort the query by the average of two attributes, something like Story.scoped.order_by('(importance + points)/2').

As I saw on MongoDB and Origin documentation, this don’t seems to be possible. Do I need to create a third attribute with the average result and order by it?

| Story           | points | importance
| first expected  | 1      | 1
| third expected  | 5      | 1
| second expected | 1      | 3

Answer 1

You can use the aggregation framework to do this. I don’t know Ruby/mongoid, but here’s how you’d do it in JavaScript:

db.coll.aggregate([
    { $project: { 
        Story: 1, 
        points: 1, 
        importance: 1, 
        avg: { $divide: [{ $add: ['$points', '$importance']}, 2]}}},
    { $sort: { avg: 1}}
    ], function(err, result){
        console.log(result);
    }
);

Outputs:

[ { _id: 50a14fcb9f0d79f4de752828,
    Story: 'first expected',
    points: 1,
    importance: 1,
    avg: 1 },
  { _id: 50a14fe59f0d79f4de75282a,
    Story: 'second expected',
    points: 1,
    importance: 3,
    avg: 2 },
  { _id: 50a14fd99f0d79f4de752829,
    Story: 'third expected',
    points: 5,
    importance: 1,
    avg: 3 } ]