CODEWITHSUNDEEP
mongodbaggregation-framework
František Jeřábek
František Jeřábek

Reputation: 663

Mongo subtract in group aggregation

I have a mongo query and I need to group and than subtract min value from max value and I have issues doing that.

Here is my query:

{
    $group : {
        _id :  {
            type: "$type",
            id : "$id"
            },
        sent : {$subtract: [{$max : "$value"}, {$min : "$value"}]}
    }
}

But I get this error message:

The $subtract accumulator is a unary operator

I would like to know how do I subtract inside the group stage.

Upvotes: 4

Views: 7713

Answers (2)

Guilherme Xavier
Guilherme Xavier

Reputation: 93

An alternative to Yathish Manjunath's answer is to use $addFields operator:

db.[collection].aggregate([   
    {
        $group : {
            _id :  {
                type: "$type",
                id : "$id"
            }, 
            maxValue : {$max : "$value"} , 
            minValue : {$min : "$value"}
        }
    },
    {
        $addFields : {
            sent : {$subtract: [ "$maxValue", "$minValue" ]}
        }
    }
])

Upvotes: 8

Yathish Manjunath
Yathish Manjunath

Reputation: 2029

Can you try the below query : 

db.[collection].aggregate([   
{
        $group : {
            _id :  {
                type: "$type",
                id : "$id"
                }, 
            maxValue : {$max : "$value"} , 
            minValue : {$min : "$value"}
        }
    },
    {
        $group : {
            _id :  {
                type: "$_id.type",
                id : "$_id.id"
                }, 
            sent : {$subtract: [ "$maxValue", "$minValue" ]}
        }
    }
 ])

Upvotes: 6

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