How to copy a value between fields within each object in an embedded array of objects in MongoDB?

Question

In my 'assemblies' collection, each document contains an embedded array of objects called 'partlist':

{
 "_id" : ObjectId("0001"),
 "pn" : "01",
 "title" : "MyAssembly",
 "partlist" : [ 
 {
    "id" : "",
    "pn" : "1234",
    "desc" : "myPart1",
 }, 
 {
    "id" : "",
    "pn" : "5678",
    "desc" : "myPart2",
 }]
}

Within each object, I need to copy the value from 'partlist.pn' into 'partlist.id'. I used:

db.assemblies.aggregate([{$set:{"partlist.id":"$partlist.pn"}}])

hoping to achieve this:

{
 "_id" : ObjectId("0001"),
 "pn" : "01",
 "title" : "MyAssembly",
 "partlist" : [ 
 {
    "id" : "1234",
    "pn" : "1234",
    "desc" : "myPart1",
 }, 
 {
    "id" : "5678",
    "pn" : "5678",
    "desc" : "myPart2",
 }]
}

Instead it returned to 'id' an array of ALL the 'pn' values in 'partlist':

{
 "_id" : ObjectId("0001"),
 "pn" : "01",
 "title" : "MyAssembly",
 "partlist" : [ 
 {
    "id" : [ 
        "1234",
        "5678"
     ],
    "pn" : "1234",
    "desc" : "myPart1",
 }, 
 {
    "id" : [ 
        "1234",
        "5678"
     ],
    "pn" : "5678",
    "desc" : "myPart2",
 }]
}

What is the correct syntax for copying the one value within each object?

It id, pn and desc all you have as fields, or are there more fields? — user12582392
– user12582392, Commented Nov 23, 2020 at 7:23

varman · Accepted Answer · 2020-11-23 07:38:32Z

1

What you can do is, you can use $map to modify each elements and mearg the id with the help of $mergeObject

db.collection.aggregate([
  {
    $addFields: {
      "partlist": {
        $map: {
          input: "$partlist",
          in: {
            "$mergeObjects": [
              "$$this",
              {
                id: "$$this.pn"
              }
            ]
          }
        }
      }
    }
  }
])

Working Mongo playground

edited Nov 23, 2020 at 7:38

answered Nov 23, 2020 at 7:25

varman

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3 Comments

user1925590 Over a year ago

Thanks varman, that did the trick. So much to learn and so little time...

varman Over a year ago

@user1925590 consider to accept the answer which may help others in future

user1925590 Over a year ago

Sure. How do I do that?

user12582392 · Accepted Answer · 2020-11-23 07:44:38Z

0

If the pn does not exist it retrieves initial object.

db.collection.aggregate([
  {
    $addFields: {
      partlist: {
        $map: {
          input: "$partlist",
          as: "p",
          in: {
            $cond: [
              "$$p.pn",
              {
                $mergeObjects: [
                  "$$p",
                  {
                    "id": "$$p.pn"
                  }
                ]
              },
              "$$p"
            ]
          }
        }
      }
    }
  }
])

Playground

answered Nov 23, 2020 at 7:44

user12582392

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