如何在Go mgo中投影其他字段

My Go code is:

  query := []bson.M{
        bson.M{
      "$match": bson.M{
        "static.created": bson.M{
          "$gt": someDate,
        },
      },
    },
    bson.M{
      "$group": bson.M{
        "_id": "$static.name",
        "count": bson.M{ "$sum": 1 },
      },
    },
    bson.M{
      "$match": bson.M{
        "count": bson.M{
          "$gte":2,
        },
      },
    },
        bson.M{
      "$project": bson.M{
        "count": 1,
        "static.name": 1,
      },
    },
  }


    pipe := c.Pipe(query)
    response := []bson.M{}
    err = pipe.All(&response)

In response I should get 3 fields: _id count static.name

But I get only two of them: _id count

How could I get additional fields in my response? I have added the field in $project but it doesn't appear in response.

I tried to remote _id field with:

bson.M{
  "$project": bson.M{
    "_id": 0,
    "count": 1,
    "static.name": 1,
  },
},

and it worked. So the $project works to remove fields but it doesn't work to add them.