[Golang] Relacionamento com GORM

gormtest.go

package main

import (
	"fmt"
	"github.com/jinzhu/gorm"
	_ "github.com/jinzhu/gorm/dialects/postgres"
)

type Manufacturer struct {
	gorm.Model
	BrandName string `json:"brand_name"`
	Products  []Product
}

type Product struct {
	gorm.Model
	Name           string `json:"name"`
	ManufacturerID uint   `sql:"manufacturer_id" json:"manufacturer_id"`
	Manufacturer   Manufacturer
}

func main() {
	URLDatabase := "postgresql://postgres:@localhost/gormtest?sslmode=disable"
	db, err := gorm.Open("postgres", URLDatabase)
	if err != nil {
		panic(err)
	}
	defer db.Close()
	db.AutoMigrate(&Manufacturer{})
	db.AutoMigrate(&Product{})

	m := Manufacturer{BrandName: "Teste Manufacturer"}
	db.Create(&m)

	p := Product{Name: "Product Test", ManufacturerID: m.ID}
	// or could be:
	// p := Product{Name: "Product Test", Manufacturer: m}
	db.Create(&p)

	product := Product{}
	db.Preload("Manufacturer").First(&product, p.ID)
	fmt.Println("Product:", product.Name)
	// => Product: Product Test
	fmt.Println("Manufacturer of product:", product.Manufacturer.BrandName)
	// => Manufacturer of product: Teste Manufacturer

	manufacturer := Manufacturer{}
	db.Preload("Products").First(&manufacturer, m.ID)
	fmt.Println("Manufacturer:", manufacturer.BrandName)
	// => Manufacturer: Teste Manufacturer
	fp := manufacturer.Products[0]
	fmt.Println("First product:", fp.Name)
	// => First product: Product Test
}

/*
// SCHEMA

gormtest=# \d manufacturers
                                    Table "public.manufacturers"
   Column   |           Type           |                         Modifiers

------------+--------------------------+------------------------------------------------------------
 id         | integer                  | not null default nextval('manufacturers_id_seq'::regclass)
 created_at | timestamp with time zone |
 updated_at | timestamp with time zone |
 deleted_at | timestamp with time zone |
 brand_name | text                     |
Indexes:
    "manufacturers_pkey" PRIMARY KEY, btree (id)
    "idx_manufacturers_deleted_at" btree (deleted_at)

gormtest=# \d products
                                      Table "public.products"
     Column      |           Type           |                       Modifiers

-----------------+--------------------------+-------------------------------------------------------
 id              | integer                  | not null default nextval('products_id_seq'::regclass)
 created_at      | timestamp with time zone |
 updated_at      | timestamp with time zone |
 deleted_at      | timestamp with time zone |
 name            | text                     |
 manufacturer_id | integer                  |
Indexes:
    "products_pkey" PRIMARY KEY, btree (id)
    "idx_products_deleted_at" btree (deleted_at)
*/