GORM don't create many2many associations - postgresql

Question

I have model:
type Book struct {
gorm.Model
Title string `json:"title"`
Author string `json:"author"`
Description string `json:"description"`
Category string `json:"Category"`
Publisher string `json:"publisher"`
AuthorsCard []*AuthorsCard `gorm:"many2many:book_authorscard; "json:"authorscard"`
}
type AuthorsCard struct {
gorm.Model
Name string `json:"name"`
Age int `json:"age"`
YearOfBirth int `json:"year"`
Biography string `json:"biography"`
}
After db.AutoMigrate(&models.Book{}, &models.AuthorsCard{})
I have a creation function:
func TestCreate() {
var testbook = models.Book{
Title: "Test",
Author: "tst",
AuthorsCard: []*models.AuthorsCard{
{
Age: 23,
Name: "test",
YearOfBirth: 1999,
Biography: "23fdgsdddTEST",
},
},
Description: "something",
}
db.Preload("AuthorsCard").Find(&models.Book{})
db.Create(&testbook)
}
despite That AuthorsCard has some data in it, I receive this as a response:
{
"ID": 78,
"CreatedAt": "2022-06-23T13:10:58.01629+03:00",
"UpdatedAt": "2022-06-23T13:10:58.01629+03:00",
"DeletedAt": null,
"title": "Test",
"author": "tst",
"description": "something",
"Category": "",
"publisher": "",
"authorscard": null
}
As you can see "authorscard" is null.
How can I save "authorscard" to the database?
I used Gorm + postgresql and also I sent a few request with postman, results are the same - Authors card is null

Answer 1

The code is working when it's called in the correct order:
func TestCreate() {
db := getDB()
db.AutoMigrate(&Book{}, AuthorsCard{})
var testbook = Book{
Title: "Test",
Author: "tst",
AuthorsCard: []*AuthorsCard{
{
Age: 23,
Name: "test",
YearOfBirth: 1999,
Biography: "23fdgsdddTEST",
},
},
Description: "something",
}
// 1. Create your testbook.
db.Create(&testbook)
// 2. Store it into a variable:
var b1 *Book
db.Preload("AuthorsCard").Find(&b1)
fmt.Println(b1.AuthorsCard[0].Age)
fmt.Println(b1.AuthorsCard[0].Name)
fmt.Println(b1.AuthorsCard[0].YearOfBirth)
fmt.Println(b1.AuthorsCard[0].Biography)
}
prints:
23
test
1999
23fdgsdddTEST
Also your JSON export might fail since you pass a pointer to AuthorCard and the marshalling not always works properly in those cases. However, GORM does the right job here.
Static check also gave me some hints here:
type Book struct {
gorm.Model
Title string `json:"title"`
Author string `json:"author"`
Description string `json:"description"`
Category string `json:"Category"`
Publisher string `json:"publisher"`
AuthorsCard []*AuthorsCard `gorm:"many2many:book_authorscard" json:"authorscard"` // wrong space
}