Has many/Belong to association with a record that already exist

[nomorechokedboy]

I have 2 struct as below. My question is how can I create a row of product with categoryId send from frontend

type Product struct {
	CategoryID  uint
	Name        string              `gorm:"size:20; not null"`
	SKU         string              `gorm:"unique;not null;size:20"`
	Price       float32             `gorm:"check:price > 0; not null"`
	Category    category.Category 
}

type Category struct {
	ID          uint    `gorm:"primaryKey"`
	Name        string  `gorm:"unique;not null;size:20"`
	Description *string `gorm:"size:1000"`
}

// This is the code I use to create a record of product

req := struct{
	Name        string  `js:"name"`
	SKU         string  `js:"sku"`
	Price       float32 `js:"price"`
	CategoryId  uint    `js:"categoryId"`
}{
        Name: "first product",
        SKU: "sku",
        Price: 123,
        CategoryId: 1,
}

product := domain.Product{
		CategoryID:  req.CategoryId,
	        Category    category.Category{Id: req.CategoryId},
		Name:        req.Name,
		SKU:         req.SKU,
		Price:       req.Price,
}
err := r.Conn.Joins(clause.Associations).Create(&product).Error

// Suppose I have created 1 category: {Id: 1, Name: "first category", Description: "desc"}

If I use the code above, it will work, but when I find one/find many, I can't use Preload nor Joins method neither, category will always be {1 "" nil} when I preload or join association