Question 368676 in Mongodb Question posted in
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Mongodb – Project a mongo db array to comman seperated string

I have the following collection

{
  "_id": "12345",
  "Products": [
    "Product1",
    "Product2"
  ],
  "State": "active",
}

And would like to project it similar to the output below

_id: "12345"
Products: "Product1, product2"

I tried this but im not getting the desired output of a comman seperated concatenated string

{
  _id: 1,
  Products: { 
      $reduce: { 
          input: "$Products", 
          initialValue:"0" ,
          in: {concat:  ["$$value", "$$this"] } 
      }
  }
}
Tags:

2

Answers


  1. 0

    You can use $reduce and $concat to concatenate the values into one string. The $cond is necessary to determine if a comma is required between the values or not.

    db.collection.aggregate([
  {
    $project: {
      "_id": 1,
      "Products": {
        $reduce: {
          input: "$Products",
          initialValue: "",
          in: {
            $concat: [
              "$$value",
              {
                $cond: [
                  {
                    $eq: [
                      "$$value",
                      ""
                    ]
                  },
                  "",
                  ","
                ]
              },
              "$$this"
            ]
          }
        }
      }
    }
  }
])

    See HERE for a working example.

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  2. 0

    This aggregation query gives the desired result:

    db.test.aggregate([{
    $project: {
        Products: {
            $ltrim: {
                input: {
                    $reduce: {
                        input: "$Products",
                        initialValue: "",
                        in: {
                            $concat: [ "$$value", ", ", "$$this" ]
                        }
                    }
                },
                chars: ", "
            }
        }
    }
}])

    The $project is used to transform the Products array of strings into a single concatenated string, using the $reduce array operator.

    Note the usage of the $ltrim operator to remove the addition comma from the concatenated string, to give the following result:

    [ 
  { 
    _id: '12345', 
     Products: 'Product1, Product2' 
  } 
]

    The projection of _id field is not explicitly required, it is by default projected.

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