How can I update a field starting with dollar in an embedded document in MongoDB?

Question

I have a document in MongoDB in a collection with a document which has a field x which value is an embedded document, created this way:

> db.c.insert({_id: 1, x: {$a: 2, b: 3}})
WriteResult({ "nInserted" : 1 })
> db.c.findOne({_id: 1})
{ "_id" : 1, "x" : { "$a" : 2, "b" : 3 } }

Note there is a field in the embedded document which starts with dollar ($a) and a field that doesn't start with dollar (b). I can update the field without dolar without problem:

> db.c.updateOne({_id: 1}, {$set: {"x.b": 30}})
{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }
> db.c.findOne({_id: 1})
{ "_id" : 1, "x" : { "$a" : 2, "b" : 30 } }

However, if I try to update the field which starts with dollar in the same way, I get an error

> db.c.updateOne({_id: 1}, {$set: {"x.$a": 20}})
WriteError({
    "index" : 0,
    "code" : 52,
    "errmsg" : "The dollar ($) prefixed field '$a' in 'x.$a' is not valid for storage.",
    "op" : {
        "q" : {
            "_id" : 1
        },
        "u" : {
            "$set" : {
                "x.$a" : 20
            }
        },
        "multi" : false,
        "upsert" : false
    }
})

Thus, how can I update a field starting with dollar in an embedded document?

I'm using MongoDB 4.4.1, in the case you need to know.

Thanks!

Answer 1

It is not possible to update field that start with $, see mongodb field name instructions,

Restrictions on Field Names:

• Field names cannot contain the null character.
• Top-level field names cannot start with the dollar sign ($) character. Otherwise, starting in MongoDB 3.6, the server permits storage of field names that contain dots (i.e. .) and dollar signs (i.e. $).

IMPORTANT

The MongoDB Query Language cannot always meaningfully express queries over documents whose field names contain these characters (see SERVER-30575).

Until support is added in the query language, the use of $ and . in field names is not recommended and is not supported by the official MongoDB drivers.