I am trying to convert id to string in aggregate function with
{
"$project": {
"_id": {
"$toString": "$_id"
}
}
}
But for some reason i am losing all other fields after aggregation and get only "_id" field. What should i change?
When you use $project on _id explicitly and with an expression or 1 then it's like selecting only the _id field.
Usually, _id is included by default so the usage is like { $project: { _id: 0 } } - the 0 means "don't include _id in the result" but include all other fields.
Read more about the behaviour of $project here.
Instead, just use $set:
{
"$set": {
"_id": {
"$toString": "$_id"
}
}
}