MongoDB get the array element count based on the condition

Question

For the given structure I need to find out the count of the likes array based on the unique slug value

{
  "_id" : ObjectId("4e8ae86d08101908e1000001"),
  "name" : ["Name"],
  "likes" : ["emp1"],
  "slug": 'slugabcd'
 }
{
  "_id" : ObjectId("4e8ae86d08101908e1000002"),
  "name" : ["Another ", "Name"],
  "likes" : ["emp1","emp2","emp4"],
  "slug": 'slugxyz'
}
{
      "_id" : ObjectId("4e8ae86d08101908e1000002"),
      "name" : ["Another ", "Name"],
      "slug": 'slugpqr'
}

Here is my code but not working

db.blog.aggregate({slug:"slugxyz"},{$project:{NumberOfItemsInArray:{$size:"likes"}}}).count();

How can we achieve this?

Will more than one document have the same slug value? If not you can just get length of array right? — Tushar Shahi
– Tushar Shahi, Commented Jun 24, 2021 at 5:03
all the docs slug should be unique, how to get the length of likes — Rijo
– Rijo, Commented Jun 24, 2021 at 5:06

Tushar Shahi · Accepted Answer · 2021-06-24 05:16:06Z

1

Without using any query stage:

db.blog.aggregate({$project:{slug : "$slug", numberOfLikes:{$size:"$likes"}}})

answered Jun 24, 2021 at 5:16

Tushar Shahi

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Rijo Over a year ago

db.blog.aggregate({$project:{slug : "$slug", numberOfLikes:{"$size":{"$ifNull":["$likes",[]]}}}}) I hope this should work more. Thanks for looking into this

h-sifat · Accepted Answer · 2021-06-24 05:27:14Z

0

Try the following:

const pipeline = [
{ 
 $group: {
    _id: "$slug",
    likesCount: {$sum: {$size: "$likes"}}
 }
]

Here we are grouping by the unique "slug" value and counting the size of "likes" array field.

answered Jun 24, 2021 at 5:27

h-sifat

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