CODEWITHSUNDEEP
mongodbaggregation-framework
Strobe_
Strobe_

Reputation: 515

MongoDB aggregation - replace field in collection from value in another collection

so i've got a collection like so

_id: "a6c67aad-e90c-4a13-aae0-74e5ca5c8632"
value : true

and one like this

_id: "a6c67aad-e90c-4a13-aae0-74e5ca5c8632"
otherValue : false

How can i use aggregation pipelines to update the second collection otherValue with the value from the first collection based on _id

I've tried using lookup and then unwind like

{
  from: 'col1', 
  localField: 'otherValue', 
  foreignField: 'value', 
  as: 'data'
}

and then unwind

{
  path: '$val'
}

But im not quite sure where to go from here, any help would be greatly appreciated.

Upvotes: 0

Views: 2288

Answers (2)

Joe
Joe

Reputation: 28316

You might use the $merge aggregation stage.

  • match the documents from the source collection that you want to use to update the second collect.
  • lookup the matching document from the second collection
  • unwind so it is a single document instead of an array (this stage also eliminates documents that don't match one from the second collection)
  • addFields to store the value from the first document into the looked up document
  • replaceRoot to the modified looked-up document
  • merge the modified documents with the original collection, matching on _id
db.collection.aggregate([
  {$match: { filter to pick the documents }},
  {$lookup: {
       from: "otherCollection"
       localField: "_id",
       foreignField: "_id",
       as: "otherDocument"
  }},
  {$unwind: "$otherDocument"},
  {$addFields:{
       "otherDocument.otherValue": "$value"
  }},
  {$replaceRoot: {newRoot: "$otherDocument"}},
  {$merge: {
       into: "otherCollection", 
       on: "_id", 
       whenMatched: "replace", 
       whenNotMatched: "insert" 
  }}
])

Upvotes: 0

Dheemanth Bhat
Dheemanth Bhat

Reputation: 4452

Try this:

db.collection1.aggregate([
    {
        $lookup: {
            from: "collection2",
            let: { c1_id: "$_id", value: "$value" },
            pipeline: [
                {
                    $match: {
                        $expr: { $eq: ["$_id", "$$c1_id"] }
                    }
                },
                {
                    $addFields: { otherValue: "$$value" }
                }
            ],
            as: "data"
        }
    },
    {
        $unwind: "$data"
    }
])

Output:

{
    "_id" : "a6c67aad-e90c-4a13-aae0-74e5ca5c8632",
    "value" : true,
    "data" : {
        "_id" : "a6c67aad-e90c-4a13-aae0-74e5ca5c8632",
        "otherValue" : true
    }
}

Where collection1 is:

{
    "_id" : "a6c67aad-e90c-4a13-aae0-74e5ca5c8632",
    "value" : true
}

Where collection2 is:

{
    "_id" : "a6c67aad-e90c-4a13-aae0-74e5ca5c8632",
    "otherValue" : false
}

Upvotes: 1

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