gistfile1.txt

http://www.mongodb.org/display/DOCS/Aggregation+Framework+-+Expression+Reference#AggregationFramework-ExpressionReference-Composition

$add:[5, $divide:[12, 2], 7]

With expressions, I can make it work properly with the nested operations enclosed in their own object:

$add:[5, { $divide:[12, 2] }, 7]


/* --------- Test --------- */

> db.test.insert({ a: 5, b: 10, c: 20, d: 12 });

> db.runCommand({ 'aggregate':'test', 'pipeline': [ { $project: { 'calculation': { $divide: [ $add: ['$a', '$b'], $add: ['$c', '$d'] ] } } } ] } );
Mon Mar 12 14:12:16 SyntaxError: missing ] after element list (shell):1

> db.runCommand({ 'aggregate':'test', 'pipeline': [ { $project: { 'calculation': { $divide: [ { $add: ['$a', '$b'] }, { $add: ['$c', '$d'] } ] } } } ] } );
{
	"result" : [
		{
			"_id" : ObjectId("4f5e620fa40e4f69fffa10b4"),
			"calculation" : 0.46875
		}
	],
	"ok" : 1
}