Go(Gorm)+MySQLなDocker環境でdial tcp connection refusedが発生した際の対処法

エラー内容

docker-compose up時に以下のエラーが発生しました。

[error] failed to initialize database, got error dial tcp 172.19.0.2:3306: connect: connection refused

原因

docker-composeでcontainer_nameを指定していなかったため

対処法

Before

docker-compose.yml

version: "3.9"
services:
  backend:
    build: .
    ports:
      - 8000:8000
    volumes:
      - .:/app
    depends_on:
      - db

  db:
    image: mysql:5.7.22
    restart: always
    environment:
      MYSQL_DATABASE: ambassador
      MYSQL_USER: root
      MYSQL_PASSWORD: root
      MYSQL_ROOT_PASSWORD: root
    volumes:
      - .dbdata:/var/lib/mysql
    ports:
      - 33066:3306

main.go

var DB *gorm.DB

func Connect() {
	var err error

	DB, err = gorm.Open(mysql.Open("root:root@tcp(db:3306)/ambassador"), &gorm.Config{})

	if err != nil {
		panic("Could not connect with database!")
	}
}

After

docker-compose.yml

container_name: godockerDBを追加

version: "3.9"
services:
  backend:
    build: .
    ports:
      - 8000:8000
    volumes:
      - .:/app
    depends_on:
      - db

  db:
    image: mysql:5.7.22
    container_name: godockerDB # 追加
    restart: always
    environment:
      MYSQL_DATABASE: ambassador
      MYSQL_USER: root
      MYSQL_PASSWORD: root
      MYSQL_ROOT_PASSWORD: root
    volumes:
      - .dbdata:/var/lib/mysql
    ports:
      - 33066:3306

main.go

godockerDBを指定

var DB *gorm.DB

func Connect() {
	var err error

	# container_nameを指定
	DB, err = gorm.Open(mysql.Open("root:root@tcp(godockerDB)/ambassador"), &gorm.Config{})

	if err != nil {
		panic("Could not connect with database!")
	}
}

参考