groupProperty not work well

I use groupProperty in withCriteria, like this:

{

                    projections {

                        groupProperty("type")

                        property("type")


                        sum("isHUPDAT")//, "numOfHUPDAT")

                        sum("duration")//, "numOfDuration")

//                        countDistinct("logicalLogID") //"count"

                    }

                }

Howerver the group not work well, I turn on the mongo profile to log execution, I found the mongo aggregation statement is

`
[ { $project: { type: 1, isHUPDAT: 1, duration: 1 } }, { $group: { _id: { type: "$type", _id: "$_id" }, sum_isHUPDAT: { $sum: "$isHUPDAT" }, sum_duration: { $sum: "$duration" } } } ]
`

Excatly, I just want to group by `type`, so correct group id should be like this ` { _id: { type: "$type" }`.

So I trace the gorm mongo source code,
In `grails-datastore-gorm-mongodb-6.1.6.RELEASE`
org.grails.datastore.mapping.mongo.query.MongoQuery
there are codes as following:

        projectProjectionHandlers.put(PropertyProjection.class, new ProjectionHandler() {

            @Override

            public String handle(PersistentEntity entity, Document projectObject, Document groupBy, PropertyProjection projection) {

                String property = projection.getPropertyName();

                projectObject.put(property, 1);

                Document id = getIdObjectForGroupBy(groupBy);

                String projectedValueKey = property.replace('.', '_');

                id.put(projectedValueKey, "$" + property);

                // we add the id to the grouping to make it not distinct

                id.put(MongoEntityPersister.MONGO_ID_FIELD, "$" + MongoEntityPersister.MONGO_ID_FIELD);

                return projectedValueKey;

            }

        });

You will alway and _id if I use group, which will make mistake.

// we add the id to the grouping to make it not distinct

                id.put(MongoEntityPersister.MONGO_ID_FIELD, "$" + MongoEntityPersister.MONGO_ID_FIELD);

I don't know why? is it a bug or how can i do ?

Start in grails-datastore-gorm-mongodb-6.1.6.RELEASE's org.grails.datastore.mapping.mongo.query.MongoQuery, especially the PropertyProjection handler and getIdObjectForGroupBy. Reproduce the withCriteria projection and compare the generated aggregation with the requested grouping. Done means groupProperty("type") produces a group key containing type without the document _id, while the sums still work.

Written by the indexing model from the issue text.