Mongodb query issue using aggregate

Question

I am struggling to write this query.

From the following dataset, I want to find out the documents who's resources array have 'EC2' and print out ONLY that element from the array.

Data:

[ 
  { 
    "projectName": "first"
    "resources": [
      {
          "resource": "EC2",
          "region": "ap-south-1",
          "params": {
              "ImageId": "ami-0bcf5425cdc1d8a85",
              "InstanceType": "t2.micro",
          }
      },
      {
          "resource": "S3",
          "region": "ap-south-1",
          "params": {
              "Bucket": "test-bucket"
          }
      }
    ],
  }
]

Expected output:

{
    "resource": "EC2",
    "region": "ap-south-1",
    "params": {
      "ImageId": "ami-0bcf5425cdc1d8a85",
      "InstanceType": "t2.micro",
    }
}

The query that I have tried :

const projects = await Project.aggregate([
      {
        $match: {
          resources: {
            $elemMatch: {
              resource: { $eq: 'EC2' },
            },
          },
        },
      },
      {
        $project: {
          resourceName: '$resources.resource',
          region: '$resources.region' // and so on
        },
      },
    ]);

^^ This is returning other elements from the resources array as well

turivishal · Accepted Answer · 2021-04-07 16:53:09Z

1

$filter to iterate loop of resources array and find matching resource
$arrayElemAt to get first matching element from above filtered result
$replaceRoot to replace above return object to root

const projects = await Project.aggregate([

  { $match: { resources: { $elemMatch: { resource: { $eq: "EC2" } } } } },
  // or below match is equal to above match condition
  // { $match: { "resources.resource": "EC2" } },

  {
    $replaceRoot: {
      newRoot: {
        $arrayElemAt: [
          {
            $filter: {
              input: "$resources",
              cond: { $eq: ["$$this.resource", "EC2"] }
            }
          },
          0
        ]
      }
    }
  }

])

Playground

edited Apr 7, 2021 at 16:53

answered Apr 7, 2021 at 16:48

turivishal

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7 Comments

Karan Kumar Over a year ago

Hi, thanks, filter worked for me. I used this $filter: { input: '$resources', as: 'each', cond: { $eq: ['$$each.resource', 'EC2'] }, }, One QUESTION, why did I have to use 2 $$ symbols? I assume first is for the document and second is to refer the as that is each variable?? CONFUSED

turivishal Over a year ago

single $ is refer to current root document' fields like you can access projectName field in filter using $projectName, and $$ refers $filter's as variable name.

Karan Kumar Over a year ago

is there a way to select the fields of the matching elements and returning them?

turivishal Over a year ago

i am not getting you exactly, can you please explain more, you can check the playground it is exactly same as your expected result.

Karan Kumar Over a year ago

I am getting the elements filtered by the filtered operator. What if I want to project specific fields of the filtered elements. Like I want to project only the params property of each filtered element.

|

Karan Kumar · Accepted Answer · 2021-04-07 17:01:08Z

1

Thanks to @turivishal

I came up with this solution:

const projects = await Project.aggregate([
      {
        $match: {
          resources: {
            $elemMatch: {
              resource: { $eq: 'EC2' },
            },
          },
        },
      },
      {
        $project: {
          resource: {
            $filter: {
              input: '$resources',
              as: 'each', // use $$ to refer
              cond: { $eq: ['$$each.resource', 'EC2'] },
            },
          },
        },
      },
    ]);

answered Apr 7, 2021 at 17:01

Karan Kumar

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