// Demonstrates how to open a change stream by using the Go driver
package main

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"os"

	"github.com/joho/godotenv"
	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
)

func main() {
	if err := godotenv.Load(); err != nil {
		log.Println("No .env file found")
	}

	var uri string
	if uri = os.Getenv("MONGODB_URI"); uri == "" {
		log.Fatal("You must set your 'MONGODB_URI' environment variable. See\n\t https://www.mongodb.com/docs/drivers/go/current/usage-examples/#environment-variable")
	}

	client, err := mongo.Connect(options.Client().ApplyURI(uri))
	if err != nil {
		panic(err)
	}
	defer func() {
		if err = client.Disconnect(context.TODO()); err != nil {
			panic(err)
		}
	}()

	// begin watch
	coll := client.Database("sample_restaurants").Collection("restaurants")

	// Creates instructions to watch for insert operations
	pipeline := mongo.Pipeline{bson.D{{"$match", bson.D{{"operationType", "insert"}}}}}

	// Creates a change stream that receives change events
	cs, err := coll.Watch(context.TODO(), pipeline)
	if err != nil {
		panic(err)
	}
	defer cs.Close(context.TODO())

	fmt.Println("Waiting For Change Events. Insert something in MongoDB!")

	// Prints a message each time the change stream receives an event
	for cs.Next(context.TODO()) {
		var event bson.M
		if err := cs.Decode(&event); err != nil {
			panic(err)
		}
		output, err := json.MarshalIndent(event["fullDocument"], "", "    ")
		if err != nil {
			panic(err)
		}
		fmt.Printf("%s\n", output)
	}
	if err := cs.Err(); err != nil {
		panic(err)
	}
	// end watch
}