GROM gorm.DB 对象剖析

// DB GORM DB definition
type DB struct {
	*Config
	Error        error
	RowsAffected int64
	Statement    *Statement
	clone        int
}

func Open(dialector Dialector, opts ...Option) (db *DB, err error)

import (
	"fmt"

	"gorm.io/driver/mysql"
	"gorm.io/gorm"
	"gorm.io/gorm/logger"
)

// MySQLConn GORM MySQL 连接。
var MySQLConn *gorm.DB

// Init gorm mysql connnection based on the configuration.
func InitMySQLConn() error {
	dsn := fmt.Sprintf("%v:%v@tcp(%v:%v)/%v?charset=utf8mb4&parseTime=True", Conf.Mysql.User, Conf.Mysql.Passwd, Conf.Mysql.IP, Conf.Mysql.Port, Conf.Mysql.Dbname)
	var err error
	MySQLConn, err = gorm.Open(mysql.Open(dsn), &gorm.Config{
		Logger: logger.Default.LogMode(logger.Info),
	})
	return err
}

// Open initialize db session based on dialector
func Open(dialector Dialector, opts ...Option) (db *DB, err error) {
	config := &Config{}

	sort.Slice(opts, func(i, j int) bool {
		_, isConfig := opts[i].(*Config)
		_, isConfig2 := opts[j].(*Config)
		return isConfig && !isConfig2
	})

	for _, opt := range opts {
		if opt != nil {
			if applyErr := opt.Apply(config); applyErr != nil {
				return nil, applyErr
			}
			defer func(opt Option) {
				if errr := opt.AfterInitialize(db); errr != nil {
					err = errr
				}
			}(opt)
		}
	}

	if d, ok := dialector.(interface{ Apply(*Config) error }); ok {
		if err = d.Apply(config); err != nil {
			return
		}
	}

	if config.NamingStrategy == nil {
		config.NamingStrategy = schema.NamingStrategy{IdentifierMaxLength: 64} // Default Identifier length is 64
	}

	if config.Logger == nil {
		config.Logger = logger.Default
	}

	if config.NowFunc == nil {
		config.NowFunc = func() time.Time { return time.Now().Local() }
	}

	if dialector != nil {
		config.Dialector = dialector
	}

	if config.Plugins == nil {
		config.Plugins = map[string]Plugin{}
	}

	if config.cacheStore == nil {
		config.cacheStore = &sync.Map{}
	}

	db = &DB{Config: config, clone: 1}

	db.callbacks = initializeCallbacks(db)

	if config.ClauseBuilders == nil {
		config.ClauseBuilders = map[string]clause.ClauseBuilder{}
	}

	if config.Dialector != nil {
		err = config.Dialector.Initialize(db)

		if err != nil {
			if db, _ := db.DB(); db != nil {
				_ = db.Close()
			}
		}
	}

	if config.PrepareStmt {
		preparedStmt := NewPreparedStmtDB(db.ConnPool)
		db.cacheStore.Store(preparedStmtDBKey, preparedStmt)
		db.ConnPool = preparedStmt
	}

	db.Statement = &Statement{
		DB:       db,
		ConnPool: db.ConnPool,
		Context:  context.Background(),
		Clauses:  map[string]clause.Clause{},
	}

	if err == nil && !config.DisableAutomaticPing {
		if pinger, ok := db.ConnPool.(interface{ Ping() error }); ok {
			err = pinger.Ping()
		}
	}

	if err != nil {
		config.Logger.Error(context.Background(), "failed to initialize database, got error %v", err)
	}

	return
}

var user User
db.First(&user, 1) // 查询 ID 为 1 的用户

tx := db.Begin()
// 在事务中执行操作
tx.Create(&User{Name: "John"})
tx.Commit() // 提交事务

var user User
db.Preload("Orders").Find(&user) // 预加载用户的订单信息

db.Callback().Create().Before("gorm:before_create").Register("update_created_at", updateCreatedAt)

type CustomType struct {
    // 自定义类型的定义
}

var custom CustomType
db.Model(&User{}).Select("custom_field").Scan(&custom)

var goods []Good
db := Db.Model(&Good{})
db.Where("name LIKE ?", "%"+name+"%")
db.Where("price >= ?", price)
db.Find(&goods)

var goods []Good
db := Db.Model(&Good{})
db.Where("name LIKE ?", "%"+name+"%")
db.Where("price < ?", price)
db.Find(&goods)

// 链式调用构建查询
result := db.Where("name = ?", "John").Order("age DESC").Find(&users)

func (db *DB) Where(query interface{}, args ...interface{}) (tx *DB) {
	tx = db.getInstance()
	if conds := tx.Statement.BuildCondition(query, args...); len(conds) > 0 {
		tx.Statement.AddClause(clause.Where{Exprs: conds})
	}
	return
}

func (db *DB) getInstance() *DB {
	if db.clone > 0 {
		tx := &DB{Config: db.Config, Error: db.Error}

		if db.clone == 1 {
			// clone with new statement
			tx.Statement = &Statement{
				DB:       tx,
				ConnPool: db.Statement.ConnPool,
				Context:  db.Statement.Context,
				Clauses:  map[string]clause.Clause{},
				Vars:     make([]interface{}, 0, 8),
			}
		} else {
			// with clone statement
			tx.Statement = db.Statement.clone()
			tx.Statement.DB = tx
		}

		return tx
	}

	return db
}