Aggregation Pipeline Stage - $group

Build the $group stage through the aggregation pipeline stage builder aggregation.NewStageBuilder using the method Group.

Suppose we want to group by age, and for each age group, we want to count how many users there are and also gather a list of names of users in each age group:

type User struct {
	mongox.Model `bson:"inline"`
	Name         string `bson:"name"`
	Age          int    `bson:"age"`
}

// mongo.Pipeline{bson.D{bson.E{Key:"$group", Value:bson.D{bson.E{Key:"_id", Value:"$age"}, bson.E{Key:"count", Value:bson.D{bson.E{Key:"$sum", Value:1}}}, bson.E{Key:"names", Value:bson.D{bson.E{Key:"$push", Value:"$name"}}}}}}}
//[
//  {
//    "$group": {
//      "_id": "$age",
//      "count": { "$sum": 1 },
//      "names": { "$push": "$name" }
//    }
//  }
//]
aggregation.NewStageBuilder().Group("$age",
    aggregation.NewBuilder().Sum("count", 1).Push("names", "$name").Build()...,
).Build()

The Group method takes two parameters: id any for specifying the field to group by, and accumulators ...bson.E for specifying the aggregation operators, such as $sum, $push, etc.