Question 229990 in Mongodb Question posted in
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Keying into external variable with Mongodb field in aggregation

Is it possible to use a document field to key into an external object in a MongoDB update using aggregation pipeline?

Here’s the idea with an object that has a single key, my actual use case will be an updateMany with an object with many ObjectIds as keys. Imagine there’s a MongoDB doc with an _id of ‘123456’:

const fooBarId = '123456'
const someObj = {}
someObj[fooBarId] = 'fooBar'

mongoApi.update(
   { _id: ObjectId(fooBarId) },
   [{ $set: { someField: someObj['$_id'] } }]
)

Currently, I see null as the value in the updated document when I’m testing this. I should see fooBar. If I can get this update working with one document, then I can apply the same logic to an updateMany where someObj has many keys of ObjectIds with values that I’d like to set the new field to individually for each.

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2

Answers


  1. Chosen as BEST ANSWER
    0

    Ended up going with this:

    const fooBarId = '123456'
const someObj = {}
someObj[fooBarId] = 'fooBar'

mongoApi.update(
        { _id: ObjectId(fooBarId) },
        [
            {
                $set: {
                    used: {
                        $let: {
                            vars: { id: { $toString: "$_id" }, extObj: someObj },
                            in: {
                                $function: {
                                    body: `function(_id, obj) { return obj[_id] }`,
                                    args: [ "$$id", "$$extObj" ],
                                    lang: "js"
                                }
                            }
                        }
                    }
                }
            }
        ]
    )

    Needed aggregation to be performant since someObj could theoretically be 5000 keys in length.


  2. 0

    One option is to use $objectToArray:

    const someObj = {
                    "649d0bae47fa0b4db69f2cf6": 1,
                    "649d0bae47fa0b4db69f2cf8": 2
                };
db.collection.updateMany(
{$expr: {$in: [{$toString: "$_id"}, Object.keys(someObj)]}},
[
  {$set: {
      someField: {$getField: {
          input: {$first: {$filter: {
                input: {$objectToArray: someObj},
                cond: {$eq: ["$$this.k", {$toString: "$_id"}]}
          }}},
          field: "v"
      }}
  }}
])

    See how it works on the playground example

    Another option is to use bulkUpdate for such cases.

    EDIT:
    For mongodb versions older than 5.0, you can use this:

    const someObj = {
                    "649d0bae47fa0b4db69f2cf6": 1,
                    "649d0bae47fa0b4db69f2cf8": 2
                };
db.collection.updateMany(
{$expr: {$in: [{$toString: "$_id"}, Object.keys(someObj)]}},
[
  {$set: {someField: {
        $first: {$reduce: {
            input: {$objectToArray: someObj},
            initialValue: [],
            in: {$concatArrays: [
                "$$value",
                {$cond: [
                    {$eq: ["$$this.k", {$toString: "$_id"}]},
                    ["$$this.v"],
                    []
                ]}
            ]}
        }}
  }}}
])
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