Merge into a nested object using mongodb aggregation-CodePudding

I have a MongoDB collection with a document structure similar to:

{
    "_id" : "xxx",
    "inner : {
        "foo" : 1,
        "bar" : 2
    }
}

Database version is 4.2.6

and an aggregation pipeline whose penultimate stage is $project with an outcome document matching something like this one:

{
    "_id" : "xxx",
    "inner : {
        "baz" : 3
    }
}

Followed by a final stage to merge into the collection:

{
    "$merge" : {
        into: "myCollection",
        on: "_id",
        whenMatched: "merge",
        whenNotMatched: "discard"
    }
}

However as a result, the existing inner document fields are overridden by the $merge results. Meaning:

Expected results:

{
    "_id" : "xxx",
    "inner : {
        "foo" : 1,
        "bar" : 2,
        "baz" : 3
    }
}

Actual results:

{
    "_id" : "xxx",
    "inner : {
        "baz" : 3
    }
}

How can I overcome this?

CodePudding user response：

The "merge" option merges the root document, so inner field is replaced in your query.
Try to replace the merge with this merge that uses pipeline.
I haven't tested it but i think it will be ok.
$$new variable is mongo-defined to refer to the document coming from pipeline.

{
    "$merge" : {
        into: "myCollection",
        on: "_id",
        whenMatched: 
          [{"$project": 
             {"_id": "$_id",
              "inner": {"$mergeObjects": ["$inner","$$new.inner"]}}}],
        whenNotMatched: "discard"
    }
}