How to calculate Product of Sum (Array fields) in mongodb?

Question

//Sample collection

db.grades.insertMany([{ _id: 1, quizzes: [ 1, 2, 3 ] },
{ _id: 2, quizzes: [ ] },
{ _id: 3, quizzes: [ 3, 8, 9 ] }])

Below is the query i am using and getting results. //Product of Sum

db.grades.aggregate(
   [
    { $project:
         { sumof:
            {
              $map:
                 {
                   input: "$quizzes",
                   as: "grade",
                   in: { $sum :[ "$$grade", "$$grade" ] }
                 }
            }
         }
      },

    {   $project :
        { "productOfSum":
            { $reduce :
                { input : '$sumof',
                    initialValue: 1,
                in : {$multiply :["$$value","$$this"]}
                }
            }
        }
    }   
    ]
)

The output of the query is as below.

{"_id":1,"productOfSum":48}
{"_id":2,"productOfSum":1}
{"_id":3,"productOfSum":1728}

Can any one advice how and why for "_id:2", value is coming as 1 even though array is null?

however the array is null. Would multiplying a null array with 1, should it result with 1 or 0? — Vijaya Bhaskar
– Vijaya Bhaskar, Commented Mar 15, 2019 at 14:04

Anirudh Simha · Accepted Answer · 2019-03-15 12:48:44Z

0

Try this: db.grades.aggregate( [

{$project:
{quizzes:
    {$cond:
        [{$eq: [{$size: "$quizzes"}, 0]} , [0], "$quizzes"] } 
    }
 },

{ $project:
     { sumof:
        {
          $map:
             {
               input: "$quizzes",
               as: "grade",
               in: { $sum :[ "$$grade", "$$grade" ] }
             }
        }
     }
  },

{   $project :
    { "productOfSum":
        { $reduce :
            { input : '$sumof',
                initialValue: 1,
            in : {$multiply :["$$value","$$this"]}
            }
        }
    }
}   
]

)

answered Mar 15, 2019 at 12:48

Anirudh Simha

3601 silver badge8 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Anirudh Simha Over a year ago

You have to initialize empty arrays with 0 value

Anirudh Simha Over a year ago

Please close your question by clicking the checkmark to the left of the answer that helped you most

Collectives™ on Stack Overflow

How to calculate Product of Sum (Array fields) in mongodb?

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related