The $add expression (as used in $project aggregation operator) supports the ability to have an explicit array of numbers to sum be specified, such as:

{$project: {total: {$add: [1, 2, 3]}}}

However, when same array numeric value is provided as an input, the addition/summation does not occur:

c.save({a: [1, 2, 3]});
assert.eq(c.aggregate({$project: {total: {$add: "$a"}}}).result[0].total, 6); // Fails

This summation is entirely contained within the document. Today, doing this requires an $unwind followed by a $group. However {$group: {_id: "$_id"}} does not stream and can easily hit the 16MB limit on large collections. Furthermore, it's a shame to use $unwind/{$group: {_id: "$_id"}}, as it's just a verbose projection.

The proposal is to make the above the test case pass.

This would need documentation updates.