mongoDB $set problem with using value of document's key/another fields value

Question

I have a problem with update and set in mongoDB.(MongoDB shell version v4.2.6) I have a collection that contains documents like:

      {
        "_id": ObjectId("5ec6b069a8640000b1002012"),
        "name": "MS WINDOWS DSP 10 HOME 32-BIT ENG",
        "brand": "MICROSOFT",
        "date_added": NumberInt("1590079220"),
        "supplier": {
            "name": "dsfgdft",
            "product_id": "00-01-901-091"
        },
    ...
}

When i try to add a new field to all the documents in the collection, with the following command:

db.products.update({},{$set:{search_stems:"$name"}},{multi:true})

The documents do not get the value of the $name for the key search_stems it just adds $name as search_stems value

     {
        "_id": ObjectId("5ec6b069a8640000b1002012"),
        "name": "MS WINDOWS DSP 10 HOME 32-BIT ENG",
        "brand": "MICROSOFT",
        "date_added": NumberInt("1590079220"),
        "supplier": {
            "name": "dsfgdft",
            "product_id": "00-01-901-091"
        },
    ...
"search_stem":"$name"
}

what am i doing wrong?

I have read the following

Update MongoDB field using value of another field

but it is not getting the direct value of a field from document they use $concat after $set

what is your MongoDB server version ? after connecting to DB just do `db.version` — whoami - fakeFaceTrueSoul, Jun 18 '20 at 16:40
Does this answer your question? [Update MongoDB field using value of another field](https://stackoverflow.com/questions/3974985/update-mongodb-field-using-value-of-another-field) that's exactly what you're looking for.. — whoami - fakeFaceTrueSoul, Jun 18 '20 at 17:00
No it is not exactly what i'm looking for i have seen that answer but i simply want to use the value of the $name ... and not to use any extra method like $concat etc... — akman, Jun 18 '20 at 17:10
that concat is for additional task as per that questions need, but the concept is same using *aggregation pipeline in updates*.. — whoami - fakeFaceTrueSoul, Jun 18 '20 at 17:18

score 1 · Accepted Answer · answered Jun 18 '20 at 17:12

Try below query which uses aggregation pipeline in .updateMany() :

db.products.updateMany({},[ { $set: { search_stems : "$name" } } ])

In aggregation we can refer to a field using $, So we're wrapping your update operation in an array [] to say it's an aggregation pipeline, Using $set operator which is an alias to $addFields in aggregation we're able to set search_stems field's value taken out from name field.

Aggregation in updates is introduced since 4.2, prior to that if you do search_stems:"$name" it means you're assigning string value $name to a field called search_stems. As a note you don't need use {multi:true} option with .update() instead use .updateMany().

Ok this solved my problem thank you for the clarification & effort — akman, Jun 18 '20 at 17:22

mongoDB $set problem with using value of document's key/another fields value

1 Answers1