Enhance database configuration to support multiple data sources

- Updated .env.example to include optional configuration for additional database instances.
- Refactored DatabaseConfig in config.go to accommodate a default database and additional instances.
- Implemented loadAdditionalDBConfigs function to dynamically load configurations for extra databases.
- Modified InitDB function in database.go to establish connections for both default and additional databases.
- Updated README.md to document the new configuration options for connecting to multiple databases.

internal/database/database.go

@@ -11,11 +11,38 @@ import (
 	"gorm.io/gorm/logger"
 )
 
-var DB *gorm.DB
+var (
+	DB            *gorm.DB
+	additionalDBs map[string]*gorm.DB
+)
 
 func InitDB() error {
 	cfg := config.AppConfig.Database
 
+	defaultDB, err := openConnection(cfg.Default)
+	if err != nil {
+		return fmt.Errorf("连接数据库失败: %v", err)
+	}
+	DB = defaultDB
+	log.Println("默认数据库连接成功")
+
+	if len(cfg.Additional) > 0 {
+		additionalDBs = make(map[string]*gorm.DB)
+		for name, instanceCfg := range cfg.Additional {
+			conn, err := openConnection(instanceCfg)
+			if err != nil {
+				return fmt.Errorf("连接数据库[%s]失败: %v", name, err)
+			}
+			additionalDBs[strings.ToLower(name)] = conn
+			log.Printf("数据库连接成功: %s\n", name)
+		}
+	} else {
+		additionalDBs = nil
+	}
+	return nil
+}
+
+func openConnection(cfg config.DatabaseInstanceConfig) (*gorm.DB, error) {
 	dsn := fmt.Sprintf("%s:%s@tcp(%s:%s)/%s?charset=utf8mb4&parseTime=True&loc=Local",
 		cfg.User,
 		cfg.Password,
@@ -24,17 +51,24 @@ func InitDB() error {
 		cfg.DBName,
 	)
 
-	var err error
-	DB, err = gorm.Open(mysql.Open(dsn), &gorm.Config{
+	db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{
 		Logger: logger.Default.LogMode(logger.Info),
 	})
-
 	if err != nil {
-		return fmt.Errorf("连接数据库失败: %v", err)
+		return nil, err
 	}
+	return db, nil
+}
 
-	log.Println("数据库连接成功")
-	return nil
+func GetAdditionalDB(name string) (*gorm.DB, bool) {
+	if name == "" || strings.EqualFold(name, "default") {
+		return DB, DB != nil
+	}
+	if additionalDBs == nil {
+		return nil, false
+	}
+	db, ok := additionalDBs[strings.ToLower(name)]
+	return db, ok
 }
 
 func AutoMigrate(models ...interface{}) error {