Golang mongodb聚合

import "go.mongodb.org/mongo-driver/mongo"
import "go.mongodb.org/mongo-driver/mongo/options"

client, err := mongo.Connect(context.TODO(), options.Client().ApplyURI("mongodb://localhost:27017"))
if err != nil {
    log.Fatal(err)
}

database := client.Database("mydb")
collection := database.Collection("mycollection")

pipeline := mongo.Pipeline{
    bson.D{{"$match", bson.D{{"field", "value"}}}},
    bson.D{{"$group", bson.D{{"_id", "$field"}, {"count", bson.D{{"$sum", 1}}}}}},
    // 添加更多的聚合阶段...
}

cursor, err := collection.Aggregate(context.TODO(), pipeline)
if err != nil {
    log.Fatal(err)
}
defer cursor.Close(context.TODO())

for cursor.Next(context.TODO()) {
    var result bson.M
    err := cursor.Decode(&result)
    if err != nil {
        log.Fatal(err)
    }
    // 处理聚合结果...
}