Add $reduce expression operator for rollup of arrays

{

    $reduce: {

        input: <array>,

        initialValue: <expression>,

        in: <expression>

    }

}

> db.coll.insert([

  {_id: 1, array_field: [1,2,3]}

  {_id: 2, array_field: []}

]);

> db.coll.aggregate([{

    $project: {

        total: {

            $reduce: {  // Use $reduce to compute the sum.

                input: "$array_field",

                initialValue: 0,

                in: {$add : ["$$value", "$$this"]}

            }

        }

    }

}])

{_id: 1, total: 6}

{_id: 2, total: 0}

// Example 2

> db.coll.insert([

  {_id: 1, array_field: [1,2,3]}

  {_id: 2, array_field: []}

]);

> db.coll.aggregate([{

  $project: {

    array_totals: {

      $reduce: {  // Compute both the sum and the product.

        input: "$array_field",

        initialValue: {sum: 0, product: 1},

        in: {

          sum: {$add : ["$$value.sum", "$$this"]},

          product: {$multiply: ["$$value.product", "$$this"]}

        }

      }

    }

  }

}])

{_id: 1, array_totals: {sum: 6, product: 6}}

{_id: 2, array_totals: {sum: 0, product: 1}}

{ _id:1, array_field: [1,2,3] }

{ _id:2, array_field: [] }

{ _id:1, array_total: 6 }

{ _id:2, array_total: 0 }

db.coll.aggregate(

   [

      { $project:

         { array_total:

            {

              $reduce:

                 {

                   $add: "$array_field",

                 }

            }

         }

      }

   ]

)