gorm: populating related field of newly created data

Question

I have the following related tables:

type Person struct {
    ID      uint64 `json:"id" gorm:"primary_key;auto_increment"`
    Name    string `json:"name"`
    Surname string `json:"surname"`
}

type Book struct {
    ID        uint64    `json:"id" gorm:"primary_key;auto_increment"`
    Title     string    `json:"title" binding:"required,min=2,max=100" gorm:"type:varchar(100)"`
    Author    Person    `json:"author" binding:"required" gorm:"foreignkey:AuthorID"` // * here
    AuthorID  uint64    `json:"-"`                                                    // * here
    WrittenIn string    `json:"written_in" gorm:"type:varchar(5)"`
    CreatedAt time.Time `json:"created_at" gorm:"default:CURRENT_TIMESTAMP"`
    UpdatedAt time.Time `json:"updated_at" gorm:"default:CURRENT_TIMESTAMP"`
}

I can successfully create data with Create() method of gorm using this function:

func CreateBook(ctx *gin.Context) {
    // validating input
    var inputData CreateBookInput

    if err := ctx.ShouldBindJSON(&inputData); err != nil {
        ctx.JSON(401, gin.H{"status": "fail", "error": err.Error()})
    }

    // create book
    book := models.Book{Title: inputData.Title, AuthorID: inputData.AuthorID, WrittenIn: inputData.WrittenIn}

    database.DB.Create(&book).Preload("Author")
    // database.DB.Preload("Author").Create(&book)
    // database.DB.Set("gorm:auto_preload", true).Create(&book)
    
    ctx.JSON(201, gin.H{"status": "success", "book": book})
}

I want to return the newly created book with its author. Expected response:

"book": {
    "id": 10,
    "title": "Chinor ostidagi duel",
    "author": {
        "id": 3,
        "name": "John",
        "surname": "Smith"
    },
    "written_in": "1983",
    "created_at": "2022-01-07T17:07:50.84473824+05:00",
    "updated_at": "2022-01-07T17:07:50.84473824+05:00"
}

But I couldn't find a way to populate related 'author'. So what I get is:

"book": {
    "id": 10,
    "title": "Chinor ostidagi duel",
    "author": {
        "id": 0,       // empty
        "name": "",    // empty
        "surname": ""  // empty
    },
    "written_in": "1983",
    "created_at": "2022-01-07T17:07:50.84473824+05:00",
    "updated_at": "2022-01-07T17:07:50.84473824+05:00"
}

Itried these methods with no success:

database.DB.Create(&book).Preload("Author")
database.DB.Preload("Author").Create(&book)
database.DB.Set("gorm:auto_preload", true).Create(&book)
database.DB.Create(&book).Set("gorm:auto_preload", true)

How can I populate related field of newly created data?

Answer 1

One possible solution that you could try is to use the AfterCreate hook.

func (b *Book) AfterCreate(tx *gorm.DB) (err error) {
    return tx.Model(b).Preload("Author").Error
}

You can find more info about hooks here.