Mongoose - $project nested array of object to root level of array

Question

My db schema looks like that:

[
  {
    title: {
      de: "Test",
      en: "test"
    },
    infos: [
      {
        label: {
          de: "Test",
          en: "test"
        },
        data: [
          {
            label: {
              de: "Test",
              en: "test"
            }
          },
          
        ],
        
      }
    ],
    
  }
]

If I use $project in model.aggregate([]) everything is working fine for objects, so it results in:

title: 'Test' when using following projection:

{title: `$title.de`,}

but for arrays of objects, it just merges all objects in an array like that:

    "infos": [
        {
            "label": [
                "Preparation",
                "Important"
            ]
        }
    ]

when using this projection:

'infos.label': `$infos.label.de`

Same for the nested array.

But the result should look like that:

{
 infos: [
  {
    label: 'Test',
    data: [
     {
       label: 'Test'
     },
    ],

  }
 ]
}

Does someone has an idea how to archive something like that?

Tom Slabbaert · Accepted Answer · 2022-09-20 09:07:52Z

1

You should just use $map to iterate over the array and change it accordingly, like so:

db.collection.aggregate([
  {
    $project: {
      title: "$title.de",
      infos: {
        $map: {
          input: "$infos",
          in: {
            $mergeObjects: [
              "$$this",
              {
                label: "$$this.label.de"
              }
            ]
          }
        }
      }
    }
  }
])

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answered Sep 20, 2022 at 9:07

Tom Slabbaert

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3 Comments

Alessandro Rhomberg Over a year ago

Thank you Tom for your awnser that works perfect for me :) I tried to apply the same logic in the nested array data array ("infos.data"). Unfortunately I get an error back. Could you tell me what a notation for the nested array would look like so that I get the same result for the respective language? ("infos.data.label")

Tom Slabbaert Over a year ago

Sure, you just need to keep nesting the same logic, like this.

saibot Over a year ago

Hey @TomSlabbaert I have another question about this topic. Do you have an idea why the syntax $project: {title: '$title.en'} is not working for deep population? When I populate and add this projection the "title" is not displayed when I add this projection: $project: {anotherTitle: "$title.en"} the "anotherTitle" is displayed in the result. Do you know if this is the wanted behaviour and do i made a mistace. This syntax is just not working for deep populate.

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