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  1. Core Server
  2. SERVER-17258

Add $reduce expression operator for rollup of arrays

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    • Fully Compatible
    • Query 13 (04/22/16)

      Syntax

      {
    $reduce: {
        input: <array>,
        initialValue: <expression>,
        in: <expression>
    }
}

      Examples

      > 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}}

      Notes

      • If input is an empty array, the value given to initialResult is returned.

      Errors

      • If a value from the input expression is anything but a single array value.
      • 'input', 'initialResult', and 'in' are required options.

      Old Description
      We currently have the $map operator to apply an expression to each element in an array and return an array with the applied results.

      Along similar lines, it would also be useful to have a $reduce operator to rollup an array into a scalar element using a specified combining function. For example, to return the sum of all array elements.

      To illustrate:

      Given an input of:

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

      We should return:

      { _id:1, array_total: 6 }
{ _id:2, array_total: 0 }

      This is currently possible using a relatively expensive $unwind and $group (plus some additionally voodoo to handle unwinding of the empty arrays), but it would be more elegant, and likely more performant, to do something like the following:

      db.coll.aggregate(
   [
      { $project:
         { array_total:
            {
              $reduce:
                 {
                   $add: "$array_field",
                 }
            }
         }
      }
   ]
)

      [Note: the above proposed syntax is just for illustrative purposes]

            Assignee:
            benjamin.murphy Benjamin Murphy
            Reporter:
            jon.rangel@mongodb.com Jon Rangel (Inactive)
            Votes:
            9 Vote for this issue
            Watchers:
            15 Start watching this issue

              Created:
              Updated:
              Resolved: