Mongo complex sorting?

Question

Hi, I know how to sort queries in MongoDB by multiple fields, e.g., db.coll.find().sort({a:1,b:-1}).

Can I sort with a user-defined function; e.g., supposing a and b are integers, by the difference between a and b (a-b)?

Thanks!

Answer 1

Why don't create the field with this operation and sort on it ?

Answer 2

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 
});