Question 174141 in Mongodb Question posted in
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Mongodb – Can I get $size in mongo aggregation right away

Exercise.aggregate([
   {$match: {}},
   {
      $group: {
         _id: '$userId',
         correct: {$push: '$stats.correctWords'} // correctWords is array => ['a', 'b', 'c']
      }
   },
   {
      $addFields: {
          correctItems: {
             $reduce: {
                 input: '$correct',
                 initialValue: [],
                 in: { $setUnion: ['$$value', '$$this'] }
             }
          }
      }
   },
   {
      $project: {
          correctsCount: {$size: {$ifNull: ['$correctItems', []]}},
      }
   }
])

some input:

{userId: 'a', stats: {correctWords: ['a', 'b'],  ...}}
{userId: 'b', stats: {correctWords: ['a', 'd'],  ...}}
{userId: 'a', stats: {correctWords: ['g', 'bc'], ...}}
{userId: 'b', stats: {correctWords: ['a', 'tt'], ...}}

need output:

[{userId: 'a', correctsCount: 4}, {userId: 'b', correctsCount: 3}]

$reduce returns an array of elements, can I somehow immediately calculate them without additional $project ?

Tags:

2

Answers


  1. 0

    You don’t need $reduce. Simply use

    {
   $set: { items: { $setUnion: "$myInputFromPreviousGroupAction" } }
}

    You can also use $size

    {
   $set: { items: { $size: { $setUnion: "$myInputFromPreviousGroupAction" } } }
}
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  2. 0

    you can calculate the count of elements directly within the $addFields stage using the $size operator and the $reduce expression.

    Exercise.aggregate([
   { $match: {} },
   {
      $group: {
         _id: '$userId',
         correct: { $push: '$stats.correctWords' }
      }
   },
   {
      $addFields: {
         correctsCount: {
            $size: {
               $reduce: {
                  input: '$correct',
                  initialValue: [],
                  in: { $setUnion: ['$$value', '$$this'] }
               }
            }
         }
      }
   }
])

    By combining the $size operator and the $reduce expression within $addFields, you can calculate the count of elements without the need for an additional $project stage.

    hope it’s helpful..

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