无法在GORM中设置“多对多”关联

Question

I'm trying to set up an association between Users and PredictionsBags. My problem is that everything works OK if I use GORM's assumed names for referring objects, but I'd like to change the names a bit.

type User struct {
    gorm.Model
    //  We’ll try not using usernames for now

    Email        string `gorm:"not null;unique_index"`
    Password     string `gorm:"-"`
    PasswordHash string `gorm:"not null"`
    Remember     string `gorm:"-"` // A user’s remember token.
    RememberHash string `gorm:"not null;unique_index"`

    Bags []PredictionsBag
}

Every user, of course, owns zero or more PredictionsBags:

type PredictionsBag struct {
    gorm.Model

    UserID uint // I want this to be "OwnerID"

    Title        string
    NotesPublic  string `gorm:"not null"` // Markdown field. May be published.
    NotesPrivate string `gorm:"not null"` // Markdown field. Only for (private) viewing and export.

    Predictions []Prediction
}

And I'd like to have .Related() work in the usual way:

func (ug *userGorm) ByEmail(email string) (*User, error) {
    var ret User

    matchingEmail := ug.db.Where("email = ?", email)

    err := first(matchingEmail, &ret)
    if err != nil {
        return nil, err
    }

    var bags []PredictionsBag
    if err := ug.db.Model(&ret).Related(&bags).Error; err != nil {
        return nil, err
    }
    ret.Bags = bags

    return &ret, nil
}

My problem is that I can't find a way to change PredictionsBag.UserID to anything else and still have GORM figure out the relationships involved. I've been reading http://gorm.io/docs/has_many.html#Foreign-Key and if I change the relevant lines to

type User struct {
    // …
    Bags []PredictionsBag `gorm:"foreignkey:OwnerID"`
}

and

type PredictionsBag struct {
    // …
    OwnerID uint
    // …
}

I get this error:

[2019-07-28 14:23:49]  invalid association [] 

What am I doing wrong? I've also been reading http://gorm.io/docs/belongs_to.html, but I'm not sure which page to follow more closely.

Answer 1

I'll have to Check Related() when I get home, but I think what you're looking for is Preload() This is my example that works with what you want.

package main

import (
    "errors"
    "fmt"
    _ "github.com/go-sql-driver/mysql"
    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/mysql"
    "log"
)

var DB *gorm.DB

func init() {
    var err error

    DB, err = gorm.Open("mysql", fmt.Sprintf("%s:%s@tcp(%s:3306)/%s?&parseTime=True&loc=Local", "root", "root", "localhost", "testing"))
    if err != nil {
        log.Fatal(err)
    }

    DB.DropTableIfExists(&User{}, &PredictionsBag{})
    DB.AutoMigrate(&User{}, &PredictionsBag{})
    user := User{Email:"[e-mail]"}
    user.Bags = append(user.Bags, PredictionsBag{OwnerID: user.ID, NotesPrivate: "1", NotesPublic: "1"})
    DB.Create(&user)
}

func main()  {

    user := User{Email:"[e-mail]"}
    err := user.ByEmail()
    if err != nil {
        log.Println(err)
    }
    fmt.Println(user.ID, user.Email, "Bags:", len(user.Bags))

    DB.Close()
}


type User struct {
    gorm.Model
    //  We’ll try not using usernames for now

    Email        string `gorm:"not null;unique_index"`
    Password     string `gorm:"-"`
    PasswordHash string `gorm:"not null"`
    Remember     string `gorm:"-"` // A user’s remember token.
    RememberHash string `gorm:"not null;unique_index"`

    Bags []PredictionsBag `gorm:"foreignkey:OwnerID"`
}

type PredictionsBag struct {
    gorm.Model
    OwnerID uint

    Title        string
    NotesPublic  string `gorm:"not null"` // Markdown field. May be published.
    NotesPrivate string `gorm:"not null"` // Markdown field. Only for (private) viewing and export.

}

func (ug *User) ByEmail() error {

    DB.Where("email = ?", ug.Email).Preload("Bags").Limit(1).Find(&ug)
    if ug.ID == 0 {
        return errors.New("no user found")
    }
    return nil
}

Using this might work with related, but I'm not sure what else needs to be changed:

    Bags []PredictionsBag `gorm:"foreignkey:OwnerID;association_foreignkey:ID"`

Update:

I can get the Related() method to work, if you state the ForeignKey like the following:

    DB.Where("email = ?", ug.Email).Limit(1).Find(&ug)
    if ug.ID == 0 {
        return errors.New("no user found")
    }
    if err := DB.Model(&ug).Related(&ug.Bags, "owner_id").Error; err != nil {
        return err
    }