CODEWITHSUNDEEP
mongodbmongodb-queryaggregation-framework
James Mitchell
James Mitchell

Reputation: 2467

MongoDB aggregate query: $size count item only if equal to certain value

I know how to write a query such that I can count the number of items in an array per document using $size.

db.collection.aggregate(
   {project : {"name" : 1, _id:0, sizeOf : {$size : "$People.Age"}}}
)

How do I write a query using $size, but I only want to count the field if it equals a certain value (for example People.Ages.Current_Age : 21). I'm assuming I'd use $eq for that?

Sample Code:

{
    "People" : [
        {
            "Name" : "Jen",
            "Ages" : {
                "Current_Age" : 32,
                "Graduate_Age" : 26,
            }
        },
        {
            "Name" : "Bill",
            "Ages" : {
                "Current_Age" : 30,
                "Graduate_Age" : 22,
            }
        },
        {
            "Name" : "Josh",
            "Ages" : {
                "Current_Age" : 27,
                "Graduate_Age" : 24,
            }
        }
    ]
}

Upvotes: 1

Views: 1399

Answers (2)

Ashh
Ashh

Reputation: 46491

You can use below aggregation

db.collection.aggregate([
  { "$project": {
    "name": 1, "_id": 0,
    "sizeOf": {
      "$size": {
        "$filter": {
          "input": "$People",
          "as": "people",
          "cond": { "$eq": ["$$people.Ages.Current_Age", 21] }
        }
      }
    }
  }}
])

Upvotes: 1

gokultp
gokultp

Reputation: 165

You can use filter operator here, filter the array before taking size.

db.collection.aggregate(
    {project : {"name" : 1, _id:0, sizeOf : {$size : {
        $filter: {
            input: "$People.Age",
            as: "age",
            cond: { $eq:[$$age, 21] }
        }
    }}}}
)

Upvotes: 0

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