join - Gorm Join がモデルにマッピングされない

Question

Preload() の代わりに SQL 結合を使用してアプリを最適化しようとしています。しかし、モデル内の結合されたテーブルが常に空であるという問題に遭遇しました。

必死になって新しいプロジェクトを開き、いくつかの簡単なサンプルコードを見つけましたが、再び同じ問題が発生しました。以下のコードを実行すると、結合されたテーブルが常に空の配列であることがわかります。

私は何か間違ったことをしていますか？

import (
"fmt"
"log"

"github.com/jinzhu/gorm"
_ "github.com/lib/pq"
)

type Language struct {
    ID   uint `gorm:"primary_key"`
    Name string
}

type Movie struct {
    ID         uint `gorm:"primary_key"`
    Title      string
    Language   Language
    LanguageID uint
}

type Artist struct {
    ID     uint `gorm:"primary_key"`
    Name   string
    Movies []Movie `gorm:"many2many:artist_movies"`
}

func createArtists() {
    langs := []Language{{Name: "english"},
        {Name: "tamil"},
        {Name: "french"}}

    for i, _ := range langs {
        if err := db.Create(&langs[i]).Error; err != nil {
            log.Fatal(err)
        }
    }

    movies := []Movie{
        {Title: "Nayagan", Language: langs[1]},
        {Title: "Anbe sivam", Language: langs[1]},
        {Title: "3 idiots", Language: langs[2]},
        {Title: "Shamithab", Language: langs[2]},
        {Title: "Dark Knight", Language: langs[0]},
        {Title: "310 to Yuma", Language: langs[0]},
    }
    for i, _ := range movies {
        if err := db.Create(&movies[i]).Error; err != nil {
            log.Fatal(err)
        }
    }

    artists := []Artist{
        {Name: "Madhavan", Movies: []Movie{movies[1],movies[2]}},
        {Name: "Kamal Hassan", Movies: []Movie{movies[0], movies[1]}},
        {Name: "Dhanush", Movies: []Movie{movies[3]}},
        {Name: "Aamir Khan", Movies: []Movie{movies[2]}},
        {Name: "Amitabh Bachchan", Movies: []Movie{movies[3]}},
        {Name: "Christian Bale", Movies: []Movie{movies[4], movies[5]}},
        {Name: "Russell Crowe", Movies: []Movie{movies[5]}},
    }

    for i, _ := range artists {
    if err := db.Create(&artists[i]).Error; err != nil {
        log.Fatal(err)
        }
    }
}

var db *gorm.DB

func main() {
    var err error
    db, err = gorm.Open("postgres", "user=admin password=1234 dbname=test sslmode=disable")
if err != nil {
    log.Fatal(err)
}
db.AutoMigrate(new(Language), new(Movie), new(Artist))
db.LogMode(true)

createArtists()

var artists []Artist

if err = db.Joins("JOIN artist_movies on artist_movies.artist_id=artists.id").
    Joins("JOIN movies on movies.id=artist_movies.movie_id").
    Joins("JOIN languages on movies.language_id=languages.id").
    Where("languages.name=?", "tamil").
    Group("artists.id").Find(&artists).Error; err != nil {
    log.Fatal(err)
}
for _, ar := range artists {
    fmt.Println(ar.Movies)
}

artists = []Artist{}
if err = db.Joins("JOIN artist_movies on artist_movies.artist_id=artists.id").
    Joins("JOIN movies on artist_movies.movie_id=movies.id").Where("movies.title=?", "Nayagan").
    Group("artists.id").Find(&artists).Error; err != nil {
    log.Fatal(err)
}

for _, ar := range artists {
    fmt.Println(ar.Movies)
}

artists = []Artist{}

if err = db.Joins("JOIN artist_movies on artist_movies.artist_id=artists.id").
    Joins("JOIN movies on artist_movies.movie_id=movies.id").
    Where("movies.title in (?)", []string{"3 idiots", "Shamitabh", "310 to Yuma"}).
    Group("artists.id").Find(&artists).Error; err != nil {
    log.Fatal(err)
}
for _, ar := range artists {
    fmt.Println(ar.Movies)
}

}