import "gorm.io/hints"

DB.Clauses(hints.Comment("select", "master")).Find(&User{})
// SELECT /*master*/ * FROM `users`;

DB.Clauses(hints.CommentBefore("insert", "node2")).Create(&user)
// /*node2*/ INSERT INTO `users` ...;

DB.Clauses(hints.CommentAfter("select", "node2")).Create(&user)
// /*node2*/ INSERT INTO `users` ...;

DB.Clauses(hints.CommentAfter("where", "hint")).Find(&User{}, "id = ?", 1)
// SELECT * FROM `users` WHERE id = ? /* hint */

plugins/gorm.go

package plugins

import (
	"fmt"

	gorm "gorm.io/gorm"
	gormclause "gorm.io/gorm/clause"
)

type Comment struct {
	Content string
}

func (c Comment) Name() string {
	return "COMMENT"
}

func (c Comment) Build(builder gormclause.Builder) {
	builder.WriteString("/* ")
	builder.WriteString(c.Content)
	builder.WriteString(" */")
}

func (c Comment) MergeClause(mergeClause *gormclause.Clause) {
}

func (c Comment) ModifyStatement(stmt *gorm.Statement) {
	clause := stmt.Clauses[c.Name()]
    // 注意这里一定要是Expression，因为Expression为nil的话，是不会触发Build方法执行的
    // 这里一开始参考hints注册的BeforeExpression，导致Build未执行，直到把整个gorm流程梳理一遍才发现问题所在
	clause.Expression = c
	stmt.Clauses[c.Name()] = clause
}

var extraClause = []string{"COMMENT"}

type CommentClausePlugin struct{}

// NewCommentClausePlugin create a new ExtraPlugin
func NewCommentClausePlugin() *CommentClausePlugin {
	return &CommentClausePlugin{}
}

// Name plugin name
func (ep *CommentClausePlugin) Name() string {
	return "CommentClausePlugin"
}

// Initialize register BuildClauses
func (ep *CommentClausePlugin) Initialize(db *gorm.DB) (err error) {
	initClauses(db)
	db.Callback().Create().Before("gorm:create").Register("CommentClausePlugin", AddAnnotation)
	db.Callback().Delete().Before("gorm:delete").Register("CommentClausePlugin", AddAnnotation)
	db.Callback().Query().Before("gorm:query").Register("CommentClausePlugin", AddAnnotation)
	db.Callback().Update().Before("gorm:update").Register("CommentClausePlugin", AddAnnotation)
	db.Callback().Raw().Before("gorm:raw").Register("CommentClausePlugin", AddAnnotation)
	db.Callback().Row().Before("gorm:row").Register("CommentClausePlugin", AddAnnotation)

	return
}

func AddAnnotation(db *gorm.DB) {
	if db.Error != nil {
		return
	}

	rid := "xx"
	// 从context上下文里取rid信息
	if v, ok := db.Statement.Context.Value("rid").(string); ok {
		rid = v
	}

	content := fmt.Sprintf("rid=%s", rid)

	if db.Statement.SQL.Len() > 0 {
		oldSQL := db.Statement.SQL.String()
		db.Statement.SQL.Reset()
		db.Statement.SQL.WriteString(fmt.Sprintf("%s %s", content, oldSQL))
		return
	}

	db.Statement.AddClause(Comment{Content: content})
}

// initClauses init SQL clause
func initClauses(db *gorm.DB) {
	if db.Error != nil {
		return
	}
	createClause := append(extraClause, db.Callback().Create().Clauses...)
	deleteClause := append(extraClause, db.Callback().Delete().Clauses...)
	queryClause := append(extraClause, db.Callback().Query().Clauses...)
	updateClause := append(extraClause, db.Callback().Update().Clauses...)
	rawClause := append(extraClause, db.Callback().Raw().Clauses...)
	rowClause := append(extraClause, db.Callback().Row().Clauses...)
	db.Callback().Create().Clauses = createClause
	db.Callback().Delete().Clauses = deleteClause
	db.Statement.Callback().Query().Clauses = queryClause
	db.Callback().Update().Clauses = updateClause
	db.Callback().Raw().Clauses = rawClause
	db.Callback().Row().Clauses = rowClause
}


main.go
package main

import (
    "context"
    "plugins"

    gorm "gorm.io/gorm"
    "github.com/google/uuid"
)


type Product struct {
  gorm.Model
  Code  string
  Price uint
}

func main() {
    dsn := "user:pass@tcp(127.0.0.1:3306)/dbname?charset=utf8mb4&parseTime=True&loc=Local"
    db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{})

    db.Use(plugins.NewCommentClausePlugin())

    db.Create(&Product{Code: "D42", Price: 100})

    // 传入context，指定rid
    ctx := context.WithValue(context.Background(), "rid", uuid.New().String())
    db.WithContext(ctx).Create(&Product{Code: "D42", Price: 100})
}