CODEWITHSUNDEEP
node.jsmongodbmongooseaggregate
Rahul Saini
Rahul Saini

Reputation: 937

How to use $subtract with array of document in mongoDB

We have two collections in which have users and organizations.

db.users.aggregate([
   {$match: {"organization.metaInfo":{"$ne": "disabled"}}},
   {"$unwind":"$organization"},
   {"$group":{"_id":"$organization.organizationId", "count":{"$sum":1}}},
   {$lookup: {from: "organisations",
              localField: "_id",
             foreignField: "_id", as: "orgDta"}},
   {"$project":{"_id":1, "count":1, 
              "orgDta.organizationLicence.userCount":1     
   }}

])

When this query is performed return a result like which is good to me.

{
  "_id" : "768d3090-d4f5-11e7-a503-9b68b90cdb4e",
  "count" : 5.0,
  "orgDta" : [{ 
           "organizationLicence" : {
              "userCount" : 50
          }
    }]
},
{
  "_id" : "d9933740-c29c-11e7-9481-b52c5f3e2e70",
  "count" : 1.0,
  "orgDta" : [{
          "organizationLicence" : {
            "userCount" : 1
        }
    }] 
 },
 {
   "_id" : "5386ebc0-c29b-11e7-9481-b52c5f3e2e70",
    "count" : 1.0,
    "orgDta" : [{
           "organizationLicence" : {
             "userCount" : 1
         }
    }]
 }

Now, I want to perform a subtract operation in between count and userCount.But I don't know that how to use here.

I was trying together with $project

{"$project":{"_id":1, "count":1, "orgDta.dObjects":1, "orgDta.organizationLicence.userCount":1, "remainingUser": { $subtract: [ "$orgDta.organizationLicence.userCount", "$count"]}

But Mongo returns error

{ "message" : "cant $subtract adouble from a array", "stack" : "MongoError: cant $subtract adouble from a array" }

Upvotes: 2

Views: 1492

Answers (1)

Nilesh Singh
Nilesh Singh

Reputation: 1750

Use $group instead wih $unwind (before) like this,

Aggregate pipeline

db.users.aggregate([
        {
            $unwind: '$orgDta'
        }, {
            $group: {
                _id: '$_id',
                remainingUser: {
                    $push: {
                        $subtract: ['$orgDta.organizationLicence.userCount', '$count']
                    }
                }
            }
        }
    ])

What we are doing here is unwind the target array, subtract all the elements (in your case, the element child's element value) and then group the array back with result (of substraction) value.

Add other items you might want in your final result document, above is just a sample MongoDB aggregate query.

Upvotes: 1

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