Gorm Polyphormic has-many association with separate association table

Your Question

I have something like this:

type MovieAmateur struct {
    ID  int   `gorm:"primary_key";autoIncrement:true;`
    Tag []Tag `gorm:"polymorphic:Movie;polymorphicValue:amatuer"`
}

type MovieProfessional struct {
    ID  int   `gorm:"primary_key";autoIncrement:true;`
    Tag []Tag `gorm:"polymorphic:Movie;polymorphicValue:professional"`
}

type Tag struct {
    ID        int    `gorm:"primary_key";autoIncrement:true;`
    MovieID   int
    MovieType string
}

This however adds two columns, movie_id and movie_type in the Tag table. So in practice it treats the entity Tag like the association table itself.

Expected answer

The table Tag in my case has a lot of extra fields, I need the association to be represented in an external table with foreign keys from Movie and Tag, e.g.

// Intermediate "bridge" table that couples together Movie* and Tag
type MovieTag struct {
    Tag       Tag `gorm:"primary_key";autoIncrement:false;`
    MovieID   int `gorm:"primary_key"`
    MovieType int `gorm:"primary_key"`
}

Is it possible with Gorm?

Comment From: resokou

I ended up linking the associations to a new table MovieTag and then manually construct the query:

package main

import (
    "log"
    "os"
    "time"
    "fmt"

    "github.com/glebarez/sqlite"
    "gorm.io/gorm"
    "gorm.io/gorm/logger"
)

type MovieAmateur struct {
    ID   int        `gorm:"primary_key";autoIncrement:true;`
    Name string
    Tags []MovieTag `gorm:"polymorphic:Movie;polymorphicValue:1000"`
}

type MovieProfessional struct {
    ID   int        `gorm:"primary_key";autoIncrement:true;`
    Name string
    Tags []MovieTag `gorm:"polymorphic:Movie;polymorphicValue:2000"`
}

type Tag struct {
    ID   int    `gorm:"primary_key";autoIncrement:true;`
    Name string
}

type MovieTag struct {
    TagID     int    `gorm:"primary_key";autoIncrement:false;`
    Tag       Tag
    MovieID   int    `gorm:"primary_key"`
    MovieType int    `gorm:"primary_key"`
}

func main() {
    dbLogger := logger.New(
        log.New(os.Stdout, "\r\n", log.LstdFlags),
        logger.Config{  // debug queries
            SlowThreshold:             time.Second,
            LogLevel:                  logger.Info,
            IgnoreRecordNotFoundError: true,
            Colorful:                  false,
        },
    )
    db, _ := gorm.Open(sqlite.Open("file::memory:?_fk=on"), &gorm.Config{Logger: dbLogger,})
    db.AutoMigrate(&Tag{}, &MovieAmateur{}, &MovieProfessional{}, &MovieTag{},)
    tag1 := Tag{Name: "Comic"}
    tag2 := Tag{Name: "Sci-Fi"}
    db.Create(&tag1)
    db.Create(&tag2)

    db.Create(&MovieAmateur{Name: "MovieA", Tags: []MovieTag{{Tag: tag1}, {Tag: tag2}},})
    db.Create(&MovieProfessional{Name: "MovieP1", Tags: []MovieTag{{Tag: tag1}, {Tag: tag2}},})
    db.Create(&MovieProfessional{Name: "MovieP2", Tags: []MovieTag{{Tag: tag2}},})

    tags := []Tag{}
    db.Table("tags").Select("*").Joins(
            "JOIN movie_tags ON tag_id==tags.id AND movie_id=1 AND movie_type=2000").Find(&tags)
    for i, tag := range tags {
        fmt.Printf("TAG %2d: %+v\n", i, tag)
    }
}

The correct combination of Movie type + Movie ID could be passed to an intermediate function (scopes?) via an Enum or by figuring it out using the reflection package I guess. But I don't know if there is an easier way to do it with GORM.

Comment From: github-actions[bot]

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Comment From: saeidee

Why you are not having the Name field inside MovieTag? here is a similar example which is having extra fields in the polymorphic table. https://gorm.io/docs/has_many.html#Polymorphism-Association