Golang – Backend (Sprint1)

contributed by < ArielWu0203 >

tags: Backend

GitHub : pd1-learning

Goal

• Set up endpoint service on localhost with port 3000
• Set up a hello world page sent back from http://localhost:3000/
• Create the Problems collection in MongoDB with Robomongo
• Create several documents with the following attributes
  • Pid (int)
  • Title (String)
  • Solution (Boolean)
  • Acceptance (Float)
• Complete the function of the endpoint GET http://localhost/api/problem that retreives the data from MongoDB with MongoDB-go-driver

Install the MongoDB Go Driver

$ cd go/src
$ go get go.mongodb.org/mongo-driver

Connect to DB

// Set client options
// options.Client() : creates a new ClientOptions instance.
clientOptions := options.Client().ApplyURI("mongodb://localhost:27017")

// Connect to MongoDB
client, err := mongo.Connect(context.TODO(), clientOptions)

if err != nil {
    log.Fatal(err)
}

// Check the connection
err = client.Ping(context.TODO(), nil)

if err != nil {
    log.Fatal(err)
}

fmt.Println("Connected to MongoDB!")

• context

  • context switch : 當使用權要從 A process 交給 B process 時，A process 的狀態需要被保存下來，記錄在 PCB 中，再載入 B process 。
  • context.Background() :
    • 已先初始化。
    • return non-nil, empty Context.
    • No canceled, No value, No deadline.
    • 通常用在 main function, top-level Context, parent Context.
  • context,TODO() :
    • 已先初始化。
    • return non-nil, empty Context.
    • 不清楚、不確定時才使用。
  • More links
    • package context
    • 上下文 Context
    • examples : Golang Context

• Collection

collection := client.Database(db_name).Collection(collection_name)

• Disconnect

err = client.Disconnect(context.TODO())

if err != nil {
    log.Fatal(err)
}
fmt.Println("Connection to MongoDB closed.")

Insert Documents

func ExampleInsertDocs(collection *mongo.Collection) {
	// Create several documents in DB
	Doc1 := Problem{1, "Zuma Game", false, 40.2}
	Doc2 := Problem{2, "Word Subsets", true, 46.2}
	Doc3 := Problem{3, "Word Search", false, 32.9}
	Docs := []interface{}{Doc1, Doc2, Doc3}

	insertResult, err := collection.InsertMany(context.TODO(), Docs)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println("Docs: ", insertResult.InsertedIDs)
}

FindOneDoc

var problem Problem
	filter := bson.M {"pid" : number}
	ctx, _ := context.WithTimeout(context.Background(), 5*time.Second)
	err := collection.FindOne(ctx, filter).Decode(&problem)
	if err != nil {
		log.Fatal(err)
	}

• BSON Objects
  • Binary-encoded JSON
  • Type :
    • D: A BSON document. This type should be used in situations where order matters, such as MongoDB commands.
    • M: An unordered map. It is the same as D, except it does not preserve order.
    • A: A BSON array.
    • E: A single element inside a D.

FindAllDocs

findOptions := options.Find()
	findOptions.SetLimit(10)
	var results []*Problem

	cur, err := collection.Find(context.TODO(), bson.D{{}}, findOptions)
	if err != nil {
		log.Fatal(err)
	}

	for cur.Next(context.TODO()) {

		var elem Problem
		err := cur.Decode(&elem)

		if err != nil {
			log.Fatal(err)
		}

		results = append(results, &elem)

	}
	if err := cur.Err(); err != nil {
		log.Fatal(err)
	}

• More Links
  • func (*Collection) Find

Reference

• GitHub : MongoDB Go Driver
• Doc : MongoDB Go Driver