多人一对一返回空

Question

I want to use gorm the use a many to one relationship in my project. My structs are like this:

type Book struct {
    ID          uint
    Title       string
    Subtitle    string
    Chapters    []Chapter `gorm:"foreignkey:BookID;association_foreignkey:ID"`

} }

// TableName is book
func (Book) TableName() string {
    return "book"
}

// Chapter of books
type Chapter struct {
    ID      uint
    BookID  string
    Chapter string
}

What I want is to get chapters of a book by using this command:book.chapters. I use the following codes to get books and chapters:

var book models.Book
    db.First(&book, "id = ?", 4)
    fmt.Println(book.ID, book.Chapters) // returns []

when I query chapters with book_id = 4, I get 11 results:

   var chapters []models.Chapter
   db.Find(&chapters, "book_id = ? ", 4)
   fmt.Println("len(chapters) = ", len(chapters)) // len(chapters) =11

and when I set db.LogMode(true) and see what happens in the code I see that querying book only queries the books not joining that with chapters:

SELECT * FROM "book" WHERE (id = 4) ORDER BY "book"."id" ASC LIMIT 1

Is there something that I missed from the documentation? how should I make the book struct the get non-empty book.chapters.

Answer 1

Your problem is probably in Preloading: http://gorm.io/docs/preload.html#Preload

Try the following:

var book models.Book
db.Preload("Chapters").First(&book, "id = ?", 4)
fmt.Println(book.ID, book.Chapters)