GORM v1 と v2 のソースコードリーディングしてみた

func Open(dialect string, args ...interface{}) (db *DB, err error)

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

// https://pkg.go.dev/gorm.io/gorm#Dialector

// Dialector GORM database dialector
type Dialector interface {
	Name() string
	Initialize(*DB) error
	Migrator(db *DB) Migrator
	DataTypeOf(*schema.Field) string
	DefaultValueOf(*schema.Field) clause.Expression
	BindVarTo(writer clause.Writer, stmt *Statement, v interface{})
	QuoteTo(clause.Writer, string)
	Explain(sql string, vars ...interface{}) string
}

func Open(dialect string, args ...interface{}) (db *DB, err error)
                          ↑ この部分

func Open(dialector Dialector, opts ...Option) (db *DB, err error)
                               ↑ この部分

type DB struct {
	sync.RWMutex
	Value        interface{}
	Error        error
	RowsAffected int64

	// single db
	db                SQLCommon
	blockGlobalUpdate bool
	logMode           logModeValue
	logger            logger
	search            *search
	values            sync.Map

	// global db
	parent        *DB
	callbacks     *Callback
	dialect       Dialect
	singularTable bool

	// function to be used to override the creating of a new timestamp
	nowFuncOverride func() time.Time
}

type SQLCommon interface {
	Exec(query string, args ...interface{}) (sql.Result, error)
	Prepare(query string) (*sql.Stmt, error)
	Query(query string, args ...interface{}) (*sql.Rows, error)
	QueryRow(query string, args ...interface{}) *sql.Row
}

type ConnPool interface {
	PrepareContext(ctx context.Context, query string) (*sql.Stmt, error)
	ExecContext(ctx context.Context, query string, args ...interface{}) (sql.Result, error)
	QueryContext(ctx context.Context, query string, args ...interface{}) (*sql.Rows, error)
	QueryRowContext(ctx context.Context, query string, args ...interface{}) *sql.Row
}

func Open(dialector Dialector, opts ...Option) (db *DB, err error) {
...
	preparedStmt := &PreparedStmtDB{
		ConnPool:    db.ConnPool,
		Stmts:       map[string]Stmt{},
		Mux:         &sync.RWMutex{},
		PreparedSQL: make([]string, 0, 100),
	}
	db.cacheStore.Store(preparedStmtDBKey, preparedStmt)

	if config.PrepareStmt {
		// db.ConnPool を prepared statement 対応版へ差し替え
		db.ConnPool = preparedStmt
	}

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

type PreparedStmtDB struct {
	Stmts       map[string]Stmt // Stmt キャッシュ
	PreparedSQL []string
	Mux         *sync.RWMutex
	ConnPool
}

type Stmt struct {
	*sql.Stmt // database/sql 標準を利用
	Transaction bool
}

// Query の場合
func (db *PreparedStmtDB) QueryContext(ctx context.Context, query string, args ...interface{}) (rows *sql.Rows, err error) {
	stmt, err := db.prepare(ctx, db.ConnPool, false, query) // ここで Stmt キャッシュを検索
	if err == nil {
		rows, err = stmt.QueryContext(ctx, args...)
		if err != nil {
			db.Mux.Lock()
			stmt.Close()
			delete(db.Stmts, query)
			db.Mux.Unlock()
		}
	}
	return rows, err
}

func (s *DB) First(out interface{}, where ...interface{}) *DB {
	newScope := s.NewScope(out)
	newScope.Search.Limit(1)

	return newScope.Set("gorm:order_by_primary_key", "ASC").
		inlineCondition(where...).callCallbacks(s.parent.callbacks.queries).db
}

func (db *DB) First(dest interface{}, conds ...interface{}) (tx *DB) {
	tx = db.Limit(1).Order(clause.OrderByColumn{
		Column: clause.Column{Table: clause.CurrentTable, Name: clause.PrimaryKey},
	})
	if len(conds) > 0 {
		if exprs := tx.Statement.BuildCondition(conds[0], conds[1:]...); len(exprs) > 0 {
			tx.Statement.AddClause(clause.Where{Exprs: exprs})
		}
	}
	tx.Statement.RaiseErrorOnNotFound = true
	tx.Statement.Dest = dest
	return tx.callbacks.Query().Execute(tx)
}

// sql の結果の出力先は destination と名付けられている模様です
out -> dest

// condition へ命名を統一している模様です
// v1 から inlineCondition 等で condition を使っているため
where -> conds

// Scope contain current operation's information when you perform any operation on the database
type Scope struct {
...

// NewScope create a scope for current operation
func (s *DB) NewScope(value interface{}) *Scope {
	dbClone := s.clone()
	dbClone.Value = value
	scope := &Scope{db: dbClone, Value: value}
	if s.search != nil {
		scope.Search = s.search.clone()
	} else {
		scope.Search = &search{}
	}
	return scope
}

func (db *DB) Limit(limit int) (tx *DB) {
	tx = db.getInstance() // この部分
	tx.Statement.AddClause(clause.Limit{Limit: limit})
	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
}

type Statement struct {
	*DB
	TableExpr            *clause.Expr
	Table                string
	Model                interface{}
	Unscoped             bool
	Dest                 interface{}
	ReflectValue         reflect.Value
	Clauses              map[string]clause.Clause
	BuildClauses         []string
	Distinct             bool
	Selects              []string // selected columns
	Omits                []string // omit columns
	Joins                []join
	Preloads             map[string][]interface{}
	Settings             sync.Map
	ConnPool             ConnPool
	Schema               *schema.Schema
	Context              context.Context
	RaiseErrorOnNotFound bool
	SkipHooks            bool
	SQL                  strings.Builder
	Vars                 []interface{}
	CurDestIndex         int
	attrs                []interface{}
	assigns              []interface{}
	scopes               []func(*DB) *DB
}

// v1 First()
func (s *DB) First(out interface{}, where ...interface{}) *DB {
	newScope := s.NewScope(out)
	newScope.Search.Limit(1)

	return newScope.Set("gorm:order_by_primary_key", "ASC").
		inlineCondition(where...).callCallbacks(s.parent.callbacks.queries).db
}

func (scope *Scope) callCallbacks(funcs []*func(s *Scope)) *Scope {
	defer func() {
		if err := recover(); err != nil {
			if db, ok := scope.db.db.(sqlTx); ok {
				db.Rollback()
			}
			panic(err)
		}
	}()
	for _, f := range funcs {
		(*f)(scope)
		if scope.skipLeft {
			break
		}
	}
	return scope
}

func Open(dialect string, args ...interface{}) (db *DB, err error) {
...
	db = &DB{
		db:        dbSQL,
		logger:    defaultLogger,
		callbacks: DefaultCallback,
		dialect:   newDialect(dialect, dbSQL),
	}
	db.parent = db

// callback.go
var DefaultCallback = &Callback{logger: nopLogger{}} // 初期化されていない..?

// callback_query.go
func init() {
	DefaultCallback.Query().Register("gorm:query", queryCallback)
	DefaultCallback.Query().Register("gorm:preload", preloadCallback)
	DefaultCallback.Query().Register("gorm:after_query", afterQueryCallback)
}

func queryCallback(scope *Scope) {
...
		// SQLDB() で gorm.SQLCommon を取得
		// gorm.SQLCommon = *sql.DB であり、標準の Query を呼び出している
		if rows, err := scope.SQLDB().Query(scope.SQL, scope.SQLVars...); scope.Err(err) == nil {
			defer rows.Close()

			columns, _ := rows.Columns()
			for rows.Next() {
				scope.db.RowsAffected++

				elem := results
				if isSlice {
					elem = reflect.New(resultType).Elem()
				}

				// 第3 引数 の []*Fields を更新してレコードの値を代入
				scope.scan(rows, columns, scope.New(elem.Addr().Interface()).Fields())
...

// v2 First()
func (db *DB) First(dest interface{}, conds ...interface{}) (tx *DB) {
	tx = db.Limit(1).Order(clause.OrderByColumn{
		Column: clause.Column{Table: clause.CurrentTable, Name: clause.PrimaryKey},
	})
	if len(conds) > 0 {
		if exprs := tx.Statement.BuildCondition(conds[0], conds[1:]...); len(exprs) > 0 {
			tx.Statement.AddClause(clause.Where{Exprs: exprs})
		}
	}
	tx.Statement.RaiseErrorOnNotFound = true
	tx.Statement.Dest = dest
	return tx.callbacks.Query().Execute(tx) // おそらくここ
}

func initializeCallbacks(db *DB) *callbacks {
	return &callbacks{
		processors: map[string]*processor{
			"create": {db: db},
			"query":  {db: db},
			"update": {db: db},
			"delete": {db: db},
			"row":    {db: db},
			"raw":    {db: db},
		},
	}
}

// gorm.Open で呼び出されている
func Open(dialector Dialector, opts ...Option) (db *DB, err error) {
...
	db.callbacks = initializeCallbacks(db)

func (p *processor) Execute(db *DB) *DB {
...
	// クエリ発行はこの部分
	for _, f := range p.fns {
		f(db)
	}

...


// セットされていた関数
// ./callbacks/query.go
func Query(db *gorm.DB) {
	if db.Error == nil {
		BuildQuerySQL(db)

		if !db.DryRun && db.Error == nil {
			rows, err := db.Statement.ConnPool.QueryContext(db.Statement.Context, db.Statement.SQL.String(), db.Statement.Vars...)
			if err != nil {
				db.AddError(err)
				return
			}
			defer rows.Close()

			gorm.Scan(rows, db, false)
		}
	}
}

func (dialector Dialector) Initialize(db *gorm.DB) (err error) {
	if dialector.DriverName == "" {
		dialector.DriverName = DriverName
	}

	// ↓ こちら
	callbacks.RegisterDefaultCallbacks(db, &callbacks.Config{
		LastInsertIDReversed: true,
	})
...

func RegisterDefaultCallbacks(db *gorm.DB, config *Config) {
...
	queryCallback := db.Callback().Query()
	queryCallback.Register("gorm:query", Query) // Query 関数をセット

// callback_query.go
func init() {
	DefaultCallback.Query().Register("gorm:query", queryCallback)
	DefaultCallback.Query().Register("gorm:preload", preloadCallback)
	DefaultCallback.Query().Register("gorm:after_query", afterQueryCallback)
}

// callbacks/query.go
func RegisterDefaultCallbacks(db *gorm.DB, config *Config) {
...
	queryCallback := db.Callback().Query()
	queryCallback.Register("gorm:query", Query)
	queryCallback.Register("gorm:preload", Preload)
	queryCallback.Register("gorm:after_query", AfterQuery)
...

db, err := gorm.Open(sqlite.Open("v2_test.db"), &gorm.Config{
	Logger: logger.Default.LogMode(logger.Info),
})

// or

// やっていることは上の実装のラッパー
db = db.Debug()