Add a field to an object in an array mongoDB

Question

I have the following schema in my mongo document:

{
  //more stuff
  arrayExample:[
    {
      //more stuff as well
      field1: "foo"
    },
    {
      //more stuff as well
      field1: "foo"
    },
    {
      //more stuff as well
      field1: "foo"
    },
    //...
  ]
}

What I want to do is add a field id: ObjectId() to each element inside the arrayExample so I could have something like follows:

    {
  //more stuff
  arrayExample:[
    {
      //more stuff as well
      field1: "foo",
      id: 1
    },
    {
      //more stuff as well
      field1: "foo",
      id: 2
    },
    {
      //more stuff as well
      field1: "foo",
      id: 3
    },
    //...
  ]
}

I tried to do this with an aggregation using map but it didn't work the way I did it. Is there a way to do this? thank you very much

hhharsha36 · Accepted Answer · 2021-06-24 18:46:55Z

1

Unfortunatunately there is no built-in way in Aggregation to generate an ObjectId (for now).

But, you can make use of the $function Operator and write a custom code to do that.

Note: The use of the $function method will impact performance significantly

Also, the $function operator will work only on MongoDB Version 4.4 and above

The below sample code will work on Mongo Shell. Convert the ObjectId() part to auto ObjectId generator by a custom JS code in the function method.

db.collection.aggregate([
  {
    "$project": {
      "arrayExample": {
        "$map": {
          "input": "$arrayExample",
          "in": {
            "$mergeObjects": [
              "$$this",
              {
                "$function": {
                  "body": "function() { return {'id': ObjectId()}; }",
                  "args": [],
                  "lang": "js",
                },
              },
            ],
          },
        }
      }
    },
  }
])

Mongo Playground Sample Execution

edited Jun 24, 2021 at 18:46

answered Jun 24, 2021 at 16:56

hhharsha36

3,3892 gold badges14 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Raf Dor Over a year ago

I noticed there is a little problem with this.. the ObjectId is the same in all objects. I did this query through Compass

hhharsha36 Over a year ago

@RafDor, You are right, I didn't think about this earlier. Apologies. I have updated the answer.

Raf Dor Over a year ago

Thank you again!

hhharsha36 Over a year ago

Glad it helped you out. Do keep in mind that this is slower. Will update the answer if I come across a more optimized solution.

Raf Dor Over a year ago

Don't worry just used it once to update some big documents. thx again

Collectives™ on Stack Overflow

Add a field to an object in an array mongoDB

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related