mongodb update operation with aggregation pipeline results in error code 14

Question

i'm trying to update the value of a field with the value of another field of a document. mongodb docs say it's possible by using an aggregation pipeline as described here.

even the sample code from the docs results in an TypeMismatch code 14 error.

command:

db.members.update(
   { },
   [
      { $set: { status: "Modified", comments: [ "$misc1", "$misc2" ] } },
      { $unset: [ "misc1", "misc2" ] }
   ],
   { multi: true }
)

result:

WriteCommandError({
    "operationTime" : Timestamp(1561779602, 1),
    "ok" : 0,
    "errmsg" : "BSON field 'update.updates.u' is the wrong type 'array', expected type 'object'",
    "code" : 14,
    "codeName" : "TypeMismatch",
    "$clusterTime" : {
        "clusterTime" : Timestamp(1561779602, 1),
        "signature" : {
            "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
            "keyId" : NumberLong(0)
        }
    }
})

is this an actual bug in mongodb or am i missing something?

Answer 1

I think what you are facing is a mongodb version issue.

According to the official documentation :

Update with Aggregation Pipeline

Starting in MongoDB 4.2, the db.collection.update() can use an aggregation pipeline for the update. The pipeline can consist of the following stages:

• $addFields and its alias $set
• $project and its alias $unset
• $replaceRoot and its alias $replaceWith.

You can see that this support is available from mongodb version 4.2, And thats why it is throwing you that error.