gorm_many2many.go

package main

import (
	"fmt"
	"log"

	"github.com/jinzhu/gorm"
	_ "github.com/jinzhu/gorm/dialects/mysql"
)

// Student table
type Student struct {
	gorm.Model
	Name     string     `gorm:"not null;index"`
	Teachers []*Teacher `gorm:"many2many:student_teacher"`
}

// Teacher table
type Teacher struct {
	gorm.Model
	Name     string     `gorm:"not null;index"`
	Students []*Student `gorm:"many2many:student_teacher"`
}

func main() {
	db, err := gorm.Open("mysql", "root:root@tcp(127.0.0.1:3306)/test?parseTime=true")
	if err != nil {
		log.Fatalln("failed to connect database", err)
	}
	defer db.Close()

	// db.Debug().AutoMigrate(new(Student), new(Teacher))

	// craete
	{
		/*
			mysql> select * from students;
			+----+---------------------+---------------------+------------+------+
			| id | created_at          | updated_at          | deleted_at | name |
			+----+---------------------+---------------------+------------+------+
			|  1 | 2019-08-18 12:41:31 | 2019-08-18 12:41:31 | NULL       | st_a |
			|  2 | 2019-08-18 12:41:31 | 2019-08-18 12:41:31 | NULL       | st_b |
			|  3 | 2019-08-18 12:41:31 | 2019-08-18 12:41:31 | NULL       | st_c |
			+----+---------------------+---------------------+------------+------+
			3 rows in set (0.00 sec)

			mysql> select * from teachers;
			+----+---------------------+---------------------+------------+------+
			| id | created_at          | updated_at          | deleted_at | name |
			+----+---------------------+---------------------+------------+------+
			|  1 | 2019-08-18 12:41:31 | 2019-08-18 12:41:31 | NULL       | t_a  |
			+----+---------------------+---------------------+------------+------+
			1 row in set (0.00 sec)

			mysql> select * from student_teacher;
			+------------+------------+
			| student_id | teacher_id |
			+------------+------------+
			|          1 |          1 |
			|          2 |          1 |
			|          3 |          1 |
			+------------+------------+
			3 rows in set (0.00 sec)
		*/
		t := Teacher{
			Name: "t_a",
			Students: []*Student{
				&Student{Name: "st_a"},
				&Student{Name: "st_b"},
				&Student{Name: "st_c"},
			},
		}
		db.Debug().Create(&t)
	}

	{
		var teacher Teacher
		db.Debug().First(&teacher, 1)

		db.Debug().Model(&teacher).Related(&teacher.Students, "Students")
		fmt.Printf("%+v", teacher)
		fmt.Println()
	}

	{

		var teacher Teacher
		db.Debug().First(&teacher, 1)

		/*
			[2019-08-18 21:03:10]  [6.94ms]  INSERT INTO `student_teacher` (`student_id`,`teacher_id`) SELECT 1,1 FROM DUAL WHERE NOT EXISTS (SELECT * FROM `student_teacher` WHERE `student_id` = 1 AND `teacher_id` = 1)
			[1 rows affected or returned ]

			[2019-08-18 21:03:10]  [5.61ms]  INSERT INTO `student_teacher` (`teacher_id`,`student_id`) SELECT 1,2 FROM DUAL WHERE NOT EXISTS (SELECT * FROM `student_teacher` WHERE `teacher_id` = 1 AND `student_id` = 2)
			[1 rows affected or returned ]

			[2019-08-18 21:03:10]  [4.42ms]  INSERT INTO `student_teacher` (`teacher_id`,`student_id`) SELECT 1,3 FROM DUAL WHERE NOT EXISTS (SELECT * FROM `student_teacher` WHERE `teacher_id` = 1 AND `student_id` = 3)
			[1 rows affected or returned ]

			[2019-08-18 21:03:10]  [1.81ms]  DELETE FROM `student_teacher`  WHERE (`student_id` NOT IN (1,2,3)) AND (`teacher_id` IN (1))
			[3 rows affected or returned ]

		*/
		db.Debug().Model(&teacher).Association("Students").Replace([]*Student{
			&Student{Model: gorm.Model{ID: 1}},
			&Student{Model: gorm.Model{ID: 2}},
			&Student{Model: gorm.Model{ID: 3}},
		})
	}
}