Aggregation Pipeline Stage - $facet

Build the $facet stage through the aggregation pipeline stage builder aggregation.NewStageBuilder using the method Facet.

Suppose we want to perform the following operations based on the User collection:

1. Calculate the total number of users.
2. Group by age and count the number of users in each age group.
3. Find the names of all users older than 18.

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

// mongo.Pipeline{bson.D{bson.E{Key:"$facet", Value:bson.D{bson.E{Key:"totalUsers", Value:mongo.Pipeline{bson.D{bson.E{Key:"$count", Value:"totalUsers"}}}}, bson.E{Key:"ageGroups", Value: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:"$sum", Value:1}}}}}}, bson.D{bson.E{Key:"$sort", Value:bson.D{bson.E{Key:"_id", Value:1}}}}}}, bson.E{Key:"over18", Value:mongo.Pipeline{bson.D{bson.E{Key:"$match", Value:bson.D{bson.E{Key:"age", Value:bson.D{bson.E{Key:"$gt", Value:[]interface {}{18}}}}}}}, bson.D{bson.E{Key:"$project", Value:bson.D{bson.E{Key:"name", Value:1}, bson.E{Key:"age", Value:1}}}}}}}}}}
// [
//  {
//    "$facet": {
//      "totalUsers": [
//     	  { "$count": "totalUsers" }
//    	],
//      "ageGroups": [
//     	  { "$group": { "_id": "$age", "count": { "$sum": 1 } } },
//        { "$sort": { "_id": 1 } }
//       ],
//      "over18": [
//        { "$match": { "age": { "$gt": 18 } } },
//        { "$project": { "name": 1, "age": 1 } }
//      ]
//    }
//  }
// ]
aggregation.NewStageBuilder().Facet(
    bsonx.NewD().
        Add("totalUsers", aggregation.NewStageBuilder().Count("totalUsers").Build()).
        Add(
            "ageGroups",
            aggregation.NewStageBuilder().
                Group("$age", aggregation.Sum("count", 1)...).
                Sort(bsonx.D("_id", 1)).Build(),
        ).
        Add(
            "over18",
            aggregation.NewStageBuilder().
                Match(aggregation.Gt("age", 18)).
                Project(bsonx.NewD().Add("name", 1).Add("age", 1).Build()).Build()).
        Build(),
).Build()