Go GORM many2many问题

Question

我找不出向模型添加关联的最佳方法。我有以下结构

type Beer struct {
        ID             uint       `json:"id"`
    Name           string     `json:"name" gorm:"not null;" sql:"unique"`
    Description    string     `json:"description" gorm:"not null;"`
    ImageURL       string     `json:"image_url"`
    AlcoholContent float64    `json:"alcohol_content, default:0"`
    Featured       bool       `json:"featured"`
    BrewStart      time.Time  `json:"brew_start"`
    BrewEnd        time.Time  `json:"brew_end"` 
    Brewers        []Brewer   `gorm:"many2many:beer_brewers" json:"brewers"`
}

type Brewer struct {
    ID        uint       `json:"id"`
    FirstName string     `json:"first_name"`
    LastName  string     `json:"last_name"`
    Title     string     `json:"title"`
    Featured  bool       `json:"featured"`
    Beers     []Beer    `gorm:"many2many:beer_brewers" json:"beers"`
}

下面是我为数据库设定种子的数据示例

Beer{
    Name:           "some pale ale",
    Description:    "a description of some pale ale",
    ImageURL:       "http://via.placeholder.com/350x150",
    AlcoholContent: 4.5,
    Featured:       false,
    BrewStart:      utils.ParseTime("30-10-2017 13:00 (AEDT)"),
    BrewEnd:        utils.ParseTime("14-11-2017 13:00 (AEDT)"),
    Brewers: []Brewer{
        Brewer{FirstName: "john", LastName: "smith", Title: "bottle shaker", Featured: false},
        Brewer{FirstName: "joe", LastName: "bloggs", Title: "bottle maker", Featured: true},
    },
},
Beer{
    Name:           "some lager",
    Description:    "a description of some pale ale",
    ImageURL:       "http://via.placeholder.com/350x150",
    AlcoholContent: 4.5,
    Featured:       false,
    BrewStart:      utils.ParseTime("30-10-2017 13:00 (AEDT)"),
    BrewEnd:        utils.ParseTime("14-11-2017 13:00 (AEDT)"),
    Brewers: []Brewer{
        Brewer{FirstName: "john", LastName: "smith", Title: "bottle shaker", Featured: false},
        Brewer{FirstName: "joe", LastName: "bloggs", Title: "bottle maker", Featured: true},
    },
},

但是，上面的代码在Brewer表中创建了重复的Brewers。我的问题是，引用已经存在的酿酒商而不是在酿酒商table?..and中创建另一个酿酒商项的最佳方式是什么？

谢谢，贾斯汀

Answer 1

尝试先创建啤酒，然后创建啤酒，并使用.append方法将啤酒添加到啤酒中。

http://jinzhu.me/gorm/associations.html#association-mode

// Start Association Mode
var user User
db.Model(&user).Association("Languages")
// `user` is the source, it need to be a valid record (contains primary key)
// `Languages` is source's field name for a relationship.
// If those conditions not matched, will return an error, check it with:
// db.Model(&user).Association("Languages").Error


// Query - Find out all related associations
db.Model(&user).Association("Languages").Find(&languages)


// Append - Append new associations for many2many, has_many, will replace current association for has_one, belongs_to
db.Model(&user).Association("Languages").Append([]Language{languageZH, languageEN})
db.Model(&user).Association("Languages").Append(Language{Name: "DE"})

Answer 2

这里的想法如下1.您独立地创建啤酒2.当您追加它们时，添加主键字段"BrewerId“，如图3.这将在brewers表中查找Brewer并将其添加到啤酒中。

import (
    "time"

    "github.com/jinzhu/gorm"

    // used by gorm
    _ "github.com/jinzhu/gorm/dialects/postgres"
)

type Beer struct {
    BeerId         uint      `json:"id"`
    Name           string    `json:"name" gorm:"not null;" sql:"unique"`
    Description    string    `json:"description" gorm:"not null;"`
    ImageURL       string    `json:"image_url"`
    AlcoholContent float64   `json:"alcohol_content, default:0"`
    Featured       bool      `json:"featured"`
    BrewStart      time.Time `json:"brew_start"`
    BrewEnd        time.Time `json:"brew_end"`
    Brewers        []Brewer  `json:"brewers" gorm:"many2many:beer_brewers;association_foreignkey:brewer_id;foreignkey:beer_id"`
}

type Brewer struct {
    BrewerId  uint   `json:"id"`
    FirstName string `json:"first_name"`
    LastName  string `json:"last_name"`
    Title     string `json:"title"`
    Featured  bool   `json:"featured"`
}

func migrate() {
    // Connection is the connection string
    connection := "host=%v port=%v user=%v dbname=%v password=%v sslmode=%v connect_timeout=%v"
    db, _ := gorm.Open("postgres", connection)
    db.AutoMigrate(&Beer{}, &Brewer{})
    db.Model(&Beer{}).Related(&Brewer{}, "Brewers")

    db.Create(&Brewer{FirstName: "justin"})
    db.Create(&Beer{
        Name:           "some lager",
    Description:    "a description of some pale ale",
    ImageURL:       "http://via.placeholder.com/350x150",
    AlcoholContent: 4.5,
    Featured:       false,
    BrewStart:      utils.ParseTime("30-10-2017 13:00 (AEDT)"),
    BrewEnd:        utils.ParseTime("14-11-2017 13:00 (AEDT)"),
    }).Association("Brewers").Append(&Brewer{BrewerId: 123,})
}

Answer 3

不确定您是否考虑过此选项-更好的方法可能是在您的Beers.Brewers字段中保留对Brewer in的引用(一片)。在对数据进行编码时，您可以将这些ID转换为完整的字段值(使用自定义的编组函数)。这可能是合适的，除非您有速度/性能方面的考虑。