重新建立与gorm Many2many 中现有记录的关联

来到golang学习网的大家，相信都是编程学习爱好者，希望在这里学习Golang相关编程知识。下面本篇文章就来带大家聊聊《重新建立与gorm Many2many 中现有记录的关联》，介绍一下，希望对大家的知识积累有所帮助，助力实战开发！

我目前正在将我的服务更新到新版本的 gorm v2。

我的多对多关联有问题，在下面的代码中，我显示了 gorm 执行的 sql 查询，我看到 grom 执行了 insert （我添加了❗️），因此当他应该只进行关联。

您知道如何在不重新创建或不修改相关元素的情况下创建关联吗？

package main

import (
    "encoding/json"
    "fmt"
    "log"
    "time"

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

type (
    // Base model
    Base struct {
        ID        string     `gorm:"type:char(36);primaryKey;" json:"id"`
        CreatedAt time.Time  `json:"created_at" diff:"-"`
        UpdatedAt time.Time  `json:"update_at" diff:"-"`
        DeletedAt *time.Time `sql:"index" json:"deleted_at" diff:"-"`
    }

    // Project model
    Project struct {
        Base
        Title   string  `json:"title" form:"title" gorm:"title;"`
        Batches []Batch `json:"batches,string" form:"batches" gorm:"batches;"`
    }

    // Batch model
    Batch struct {
        Base
        Title     string     `json:"title" form:"title" gorm:"title;"`
        Project   Project    `json:"project"`
        ProjectID string     `json:"project_id" form:"project_id" gorm:"type:char(36);project_id;"`
        Materials []Material `json:"materials" gorm:"many2many:batches_materials;association_foreignkey:ID;foreignkey:ID"`
    }

    // Material model
    Material struct {
        Base
        Title      string   `json:"title" form:"title" gorm:"title;"`
        Category   Category `json:"category"`
        CategoryID string   `json:"category_id" form:"category_id" gorm:"type:char(36);category_id;"`
        Batches    []Batch  `json:"batches" gorm:"many2many:batches_materials;association_foreignkey:ID;foreignkey:ID"`
    }

    // Category model
    Category struct {
        Base
        Name string `json:"name" form:"name" gorm:"name;"`
    }
)

// BeforeCreate will set a UUID rather than numeric ID.
func (b *Base) BeforeCreate(tx *gorm.DB) (err error) {
    uuid := uuid.New()
    b.ID = uuid.String()
    return
}

func main() {
    url := fmt.Sprintf("postgres://%v:%v@%v:%v/%v?sslmode=disable", "postgres", "postgres_docker", "0.0.0.0", "5432", "test_gorm")
    db, _ := gorm.Open(postgres.Open(url), &gorm.Config{})
    db.Migrator().DropTable(&Batch{}, &Project{}, &Material{}, &Category{})
    db.AutoMigrate(&Batch{}, &Project{}, &Material{}, &Category{})

    db = db.Debug()

    // Create Project
    project := Project{
        Title: "Project 1",
    }
    db.Save(&project)
    // ✅ INSERT INTO "projects" ("id","created_at","updated_at","deleted_at","title") VALUES ('92db2acd-7d71-41dc-bb28-07a06c25fd79','2020-11-21 07:03:01.61','2020-11-21 07:03:01.61',NULL,'Project 1')

    // Create Category
    category := Category{
        Name: "Category 1",
    }
    db.Save(&category)
    // ✅ INSERT INTO "categories" ("id","created_at","updated_at","deleted_at","name") VALUES ('fdd433f2-9153-4ad8-b4e0-da20f69994c9','2020-11-21 07:03:01.615','2020-11-21 07:03:01.615',NULL,'Category 1')

    // Create Material 1
    material1 := Material{
        Title:      "Material 1",
        CategoryID: category.ID,
    }
    db.Save(&material1)
    // ✅ INSERT INTO "materials" ("id","created_at","updated_at","deleted_at","title","category_id") VALUES ('a8beeac9-1cbb-487a-bdad-50bc70ea6f09','2020-11-21 07:03:01.621','2020-11-21 07:03:01.621',NULL,'Material 1','fdd433f2-9153-4ad8-b4e0-da20f69994c9')

    // Create Material 2
    material2 := Material{
        Title:      "Material 2",
        CategoryID: category.ID,
    }
    db.Save(&material2)
    // ✅ INSERT INTO "materials" ("id","created_at","updated_at","deleted_at","title","category_id") VALUES ('a940ac32-d5e0-4ac8-9f5f-f0bfc54e52ab','2020-11-21 07:03:01.627','2020-11-21 07:03:01.627',NULL,'Material 2','fdd433f2-9153-4ad8-b4e0-da20f69994c9')

    // Create Batch
    batch := Batch{
        Title:     "Batch 1",
        ProjectID: project.ID,
    }
    db.Save(&batch)
    // ✅ INSERT INTO "batches" ("id","created_at","updated_at","deleted_at","title","project_id") VALUES ('d48546f8-7cc7-4523-8910-72b02744a152','2020-11-21 07:03:01.631','2020-11-21 07:03:01.631',NULL,'Batch 1','92db2acd-7d71-41dc-bb28-07a06c25fd79')

    // Now i want to link Materials of batch

    // Find Batch
    myBatch := Batch{}
    db.Where("id = ?", batch.ID).First(&myBatch)
    // ✅ SELECT * FROM "batches" WHERE id = 'd48546f8-7cc7-4523-8910-72b02744a152' ORDER BY "batches"."id" LIMIT 1

    // Find two Materials
    materials := []Material{}
    db.Where("id in (?)", []string{material1.ID, material2.ID}).Find(&materials)
    // ✅ SELECT * FROM "materials" WHERE id in ('a8beeac9-1cbb-487a-bdad-50bc70ea6f09','a940ac32-d5e0-4ac8-9f5f-f0bfc54e52ab')

    // Add two Materials Association for batch model
    db.Model(&myBatch).Association("Materials").Replace(&materials)
    // ❗️ INSERT INTO "materials" ("id","created_at","updated_at","deleted_at","title","category_id") VALUES ('f2629f9d-403f-425d-afc5-074b5e8a0ecc','2020-11-21 07:03:01.621','2020-11-21 07:03:01.621',NULL,'Material 1','fdd433f2-9153-4ad8-b4e0-da20f69994c9'),('d5b709d6-1208-4951-887a-2cfafab75774','2020-11-21 07:03:01.627','2020-11-21 07:03:01.627',NULL,'Material 2','fdd433f2-9153-4ad8-b4e0-da20f69994c9') ON CONFLICT DO NOTHING
    // ✅ INSERT INTO "batches_materials" ("batch_id","material_id") VALUES ('d48546f8-7cc7-4523-8910-72b02744a152','f2629f9d-403f-425d-afc5-074b5e8a0ecc'),('d48546f8-7cc7-4523-8910-72b02744a152','d5b709d6-1208-4951-887a-2cfafab75774') ON CONFLICT DO NOTHING
    // ✅ DELETE FROM "batches_materials" WHERE "batches_materials"."batch_id" = 'd48546f8-7cc7-4523-8910-72b02744a152' AND "batches_materials"."material_id" NOT IN ('f2629f9d-403f-425d-afc5-074b5e8a0ecc','d5b709d6-1208-4951-887a-2cfafab75774')
    
    d, _ := json.MarshalIndent(batch, " ", "\t")
    log.Println(string(d))
}

解决方案

我使用 gen_random_uuid() 的本机 postgresql 函数的潜力修复了该问题，但牺牲了 beforecreate。

type Base struct {
    ID          string     `json:"id" gorm:"type:uuid;primaryKey;default:gen_random_uuid()"` // <-- like this
    CreatedAt   time.Time  `json:"created_at" diff:"-"`
    UpdatedAt   time.Time  `json:"update_at" diff:"-"`
    DeletedAt   *time.Time `sql:"index" json:"deleted_at" diff:"-"`
    Displayname string     `json:"displayname" gorm:"displayname;" diff:"-"`
}

到这里，我们也就讲完了《重新建立与gorm Many2many 中现有记录的关联》的内容了。个人认为，基础知识的学习和巩固，是为了更好的将其运用到项目中，欢迎关注golang学习网公众号，带你了解更多关于的知识点！