Join array elements using MongoDB aggregation framework

Question

I have an array of data returned by MongoDB. I would like to know how to join first name and last name from the below array using mongodb aggregation framework.

i have seen couple of posts similer to my query,however i did not understand the answer and hence i am posting a new question.

I have written a sample code.any helping corrcting my code would be really appreciated

var playersData = [
    {
        firstName: 'David',
        LastName: 'John',
        country: 'India'
    },
    {
        firstName: 'Chris',
        LastName: 'Jericho',
        country: 'USA'
    },
    {
        firstName: 'John',
        LastName: 'Cena',
        country: 'USA'
    }
];

code

playerModel.aggregate([
        {
            "$match": {
               [{ "country": 'USA' }]
            }
        },
        {
            "$project": {
                "_id": 0, "playersData.firstName": 1, "playersData.lastName": 1, 
                fullName: {
                    $reduce: {
                        input: '$playersData',
                        initialValue: '',
                        in: {
                            $concat: ["$$value", "$$this"]
                        }
                    }
                }   

            }
        }

    ], function (err, result) {

    })

Answer 1

You can try below aggregation.

$match to keep player array where at least one of the country is USA followed by $filter to filter elements with country as USA and $map with $concat to join the first name and last name to produce the full name array.

playerModel.aggregate([
  {
    "$match": {
      "playersData.country": "USA"
    }
  },
  {
    "$project": {
      "_id": 0,
      "playersData.firstName": 1,
      "playersData.lastName": 1,
      "fullname": {
        "$map": {
          "input": {
            "$filter": {
              "input": "$playersData",
              "as": "playerf",
              "cond": {
                "$eq": [
                  "$$playerf.country", "USA"
                ]
              }
            }
          },
          "as": "playerm",
          "in": {
            "$concat": [
              "$$playerm.firstName",
              ",",
              "$$playerm.lastName"
            ]
          }
        }
      }
    }
  }
])