MongoDB $concat values of array with other fields?

Question

I need to concat values of an array with some other fields.

Given is the following dataset :

{
  _id: ObjectId("1234"),
  field1: "test1",
  field2: "test2",  
  collection2: ['apple', 'banana', "orange"]  
}

I want this result :

{
  _id:ObjectId("1234")
  fruits:"test1 apple banana orange"
}

I got this so far (in $project stage) :

{
  'fruits': {
    '$concat': [
      '$field1', ' ',
      '$reduce': {
          'input': '$fruits',
          'initialValue': '',
          'in': {
              '$concat': [
                  '$$value',
                  {'$cond': [{'$eq': ['$$value', '']}, '', ' ']}, 
                  '$$this']
          }
      }
    ]
  }
}

I don't really understand why this isn't working, any idea ?

Thanks

You are missing the wrapping {} around the $reduce in order to be a valid object expression. You should be getting an error like that from the mongo shell. Also just input: { $concatArrays: [ ['$field1'], '$fruits'] } using just the $reduce is much cleaner than nesting the result within another $concat. — Neil Lunn
– Neil Lunn, Commented Mar 16, 2019 at 21:47

Jitendra · Accepted Answer · 2019-03-16 20:41:05Z

1

Try something like below:

db.fruits.aggregate([
    {
        $project: {
            values: {
                $reduce: {
                    input: '$collection2',
                    initialValue: '',
                    in: {
                        $concat: ['$$value',' ','$$this']
                    }
                }
            },
            field1: 1
        }
    },
    {
        $project: {
            'fruits': { '$concat': [ '$field1', '$values'] }
        }
    }
    ])

answered Mar 16, 2019 at 20:41

Jitendra

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