Is it possible to rename _id field after mongo's group aggregation?

I have a query like this (simplified):

db.collection.aggregate([
  { $match: { main_id: ObjectId("58f0f67f50c6af16709fd2c7") } }, 
  {
    $group: {
      _id: "$name",
      count: { $sum: 1 },
      sum: { $sum: { $add: ["$P31", "$P32"] } }
    }
  }
])

I do this query from Java, and I want to map it on my class, but I don't want _id to be mapped on name field. Because if I do something like this:

@JsonProperty("_id")
private String name;

then when I save this data back to mongo (after some modification) the data is saved with name as _id while I want a real Id to be generated.

So, how can I rename _id after $group operation?

You can achieve this by adding a $project stage at the end of your pipeline like this :

{ $project: {  
      _id: 0,
      name: "$_id",
      count: 1,
      sum: 1
   }
}

try it online: mongoplayground.net/p/QpVyh-0I-bP

From mongo v3.4 you could use $addFields in conjunction with $project to avoid to write all the fields in $project that could be very tedious.

This happen in $project because if you include specifically a field, the other fields will be automatically excluded.

Example:

{ 
  $addFields: { my_new_id_name: "$_id" }
},
{
  $project: { _id: 0 }
}

 db.report.aggregate(   
{     
$group: {_id: '$name'} 
},
{
$project:{
  name:"$_id",
 _id:false} }
 )

if you are using find method you can't do this, but if you using aggregation it is very easy like this:

db.collectionName.aggregate([
    {
        $project: {
            newName: "$existingKeyName"
        }
    }
]);

Starting in Mongo 4.2, you can use a combination of $set / $unset stages:

// { x: 1, z: "a" }
// { x: 2, z: "b" }
db.collection.aggregate([  { $set: { y: "$x" } },  { $unset: "x" }])
// { y: 1, z: "a" }
// { y: 2, z: "b" }

The $set stage adds the new field to documents and the $unset stage removes/excludes the field to be renamed from documents.

As all of the answers are written the solution in MongoDB query despite the question seeks the solution in Java, posting my approach using Java for posterities.

After the grouping, we can rename the _id fieldname using Projections.computed("<expected field name>", "$_id")))

To Transform the core part of the query mentioned in the question to Java

        Bson mainIdMatch = match(eq("main_id", new ObjectId("58f0f67f50c6af16709fd2c7")));
        Bson group = Aggregates.group("$name", Accumulators.sum("count", 1L));
        Bson project = Aggregates.project(Projections.fields(Projections.excludeId(),
                Projections.computed("name", "$_id")));
        reportMongoCollection.aggregate(Arrays.asList(mainIdMatch, group, project))
                .into(new ArrayList<>());

To answer specifically, I have added an excerpt from the above code snippet, where I am renaming _id field value as name using Projections.computed("name", "$_id") which map the values of _id which we got as a result of grouping to the field called name. Also, we should exclude the id using Projections.excludeId().

Aggregates.project(Projections.fields(Projections.excludeId(),
                Projections.computed("name", "$_id")))