Convert field from list of objectids to list of string

I have this pipeline:

            {"$lookup": {
                "from": "base_model",
                "localField": "suboptions",
                "foreignField": "_id",
                "as": "suboptions"
            {"$project": {"name": "$name",
                          "uuid": "$uuid",
                          "is_required": "$is_required",
                          "is_suboption": "$is_subption",
                          "suboptions": "$suboptions",
                          "option_uuid": "$option_uuid",
                          "is_in_stock": "$is_in_stock",
                          "created_at": "$created_at",
                          "updated_at": "$updated_at",
                          "_id": 0,
            {"$addFields": {
                "": {"$toString": "$suboptions._id"}
            {"$project": {
                "suboptions._id": 0,


If I remove the $addFields stage it works fine, but I need to get the result in JSON serializable format, so I need to change the type of _id field in suboptions to string, but this is returning Unsupported conversion from array to string in $convert with no onError value.
How can I fix this?

The result of a lookup, field suboptions, is an array not a string.

Most likely, you will need to use