CODEWITHSUNDEEP
mongodbmongodb-queryaggregation-framework
Dmitry Mubarakshin
Dmitry Mubarakshin

Reputation: 75

MongoDB v3.4 initialize new field value based on array's value of each record

I have a collection with the following structure:

{
    "_id":NUUID(...),
    "MyArray": [{
        "Name": "...",
        "MyId": "..."
    },
    {...}]
},
{...}

My task is to introduce a new field ArrayId which has to contain value from MyId if MyArray.Name is X and contain nothing (empty string) otherwise.

I tried the following:

db.getCollection('myCollection').aggregate([
    {$addFields: {"ArrayId": {$cond: {if: {$eq: ["$MyArray.Name", "X"]}, then: "$MyArray.MyId", else: ""}}}},
    {$out: "myCollection_backup"}
])

but the resulting collection has empty ArrayId for all records.

Can you please help find the error? Is there an easier way to do this in mongo 3.4?

Upvotes: 1

Views: 37

Answers (1)

R2D2
R2D2

Reputation: 10727

Maybe something like this:

db.collection.aggregate([
{
 $addFields: {
  "ArrayId": {
    "$reduce": {
      "input": "$MyArray",
      "initialValue": "",
      "in": {
        $concat: [
          "$$value",
          {
            "$cond": {
              if: {
                $eq: [
                  "$$this.Name",
                  "X"
                ]
              },
              then: "$$this.MyId",
              else: ""
            }
          }
        ]
      }
    }
  }
}
}
])

Explained:

Use $addFileds/$reduce to $concat the MyArray.MyId into the new variable SArrayId in case MyArray.Name="X"

( This is in case we expect only one of MyArray.Name=X , what is expected to happen in case there is more then 1x MyArray.Name=X ? )

Playground

Here is version updating only when it finds first Name=X(in case there is more then one Name=X):

Playground2

Upvotes: 1

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