Mongo complex sorting?

UPDATE: This answer appears to be out of date; it seems that custom sorting can be more or less achieved by using the $project function of the aggregation pipeline to transform the input documents prior to sorting. See also @Ari's answer.

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I don't think this is possible directly; the sort documentation certainly doesn't mention any way to provide a custom compare function.

You're probably best off doing the sort in the client, but if you're really determined to do it on the server you might be able to use db.eval() to arrange to run the sort on the server (if your client supports it).

Server-side sort:

db.eval(function() { 
  return db.scratch.find().toArray().sort(function(doc1, doc2) { 
    return doc1.a - doc2.a 
  }) 
});

Versus the equivalent client-side sort:

db.scratch.find().toArray().sort(function(doc1, doc2) { 
  return doc1.a - doc2.b 
});

Note that it's also possible to sort via an aggregation pipeline and by the $orderby operator (i.e. in addition to .sort()) however neither of these ways lets you provide a custom sort function either.

Ran into this and this is what I came up with:

db.collection.aggregate([
  { 
    $project: {
      difference: { $subtract: ["$a", "$b"] }
      // Add other keys in here as necessary
    }
  },  
  { 
    $sort: { difference: -1 } 
  }
])