use callbacks.Begin as a hook to handle begin operation

make transaction support hooks

callbacks.go

@@ -79,19 +79,10 @@ func (cs *callbacks) Transaction() *processor {
 }
 
 func (p *processor) Begin(tx *DB, opt *sql.TxOptions) *DB {
-	var err error
+	tx.InstanceSet("gorm:transaction_options", opt)
 
-	switch beginner := tx.Statement.ConnPool.(type) {
-	case TxBeginner:
-		tx.Statement.ConnPool, err = beginner.BeginTx(tx.Statement.Context, opt)
-	case ConnPoolBeginner:
-		tx.Statement.ConnPool, err = beginner.BeginTx(tx.Statement.Context, opt)
-	default:
-		err = ErrInvalidTransaction
-	}
-
-	if err != nil {
-		_ = tx.AddError(err)
+	for _, f := range p.fns {
+		f(tx)
 	}
 
 	return tx

callbacks/callbacks.go

@@ -80,4 +80,7 @@ func RegisterDefaultCallbacks(db *gorm.DB, config *Config) {
 	rawCallback := db.Callback().Raw()
 	rawCallback.Register("gorm:raw", RawExec)
 	rawCallback.Clauses = config.QueryClauses
+
+	transactionCallback := db.Callback().Transaction()
+	transactionCallback.Register("gorm:begin", Begin)
 }

callbacks/transaction.go

@@ -1,6 +1,8 @@
 package callbacks
 
 import (
+	"database/sql"
+
 	"gorm.io/gorm"
 )
 
@@ -30,3 +32,33 @@ func CommitOrRollbackTransaction(db *gorm.DB) {
 		}
 	}
 }
+
+func Begin(tx *gorm.DB) {
+	err := tx.Error
+
+	if err != nil {
+		return
+	}
+
+	var opt *sql.TxOptions
+
+	if v, ok := tx.InstanceGet("gorm:transaction_options"); ok {
+		if txOpts, ok := v.(*sql.TxOptions); ok {
+			opt = txOpts
+		}
+	}
+
+	switch beginner := tx.Statement.ConnPool.(type) {
+	case gorm.TxBeginner:
+		tx.Statement.ConnPool, err = beginner.BeginTx(tx.Statement.Context, opt)
+	case gorm.ConnPoolBeginner:
+		tx.Statement.ConnPool, err = beginner.BeginTx(tx.Statement.Context, opt)
+	default:
+		err = gorm.ErrInvalidTransaction
+	}
+
+	if err != nil {
+		_ = tx.AddError(err)
+	}
+
+}