Golang Gorm one-to-many with has-one

As described in this issue, gorm is not designed to use joins to preload other structs values. If you would like to continue to use gorm and have the ability to use joins to load values, one must use the SQL Builder exposed in gorm, and write some code to scan the desired values.

This would become burdensome if there are numerous tables that have to be accounted for. If xorm is available as an option, they support loading struct values. Described under the find bullet point, here.

Note: I did not scan all the fields, just enough to get the point across.

EXAMPLE:

package main

import (
    "log"

    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/sqlite"
    "github.com/kylelemons/godebug/pretty"
)

// Order
type Order struct {
    gorm.Model
    Status     string
    OrderItems []OrderItem
}

// Order line item
type OrderItem struct {
    gorm.Model
    OrderID  uint
    ItemID   uint
    Item     Item
    Quantity int
}

// Product
type Item struct {
    gorm.Model
    ItemName string
    Amount   float32
}

var (
    items = []Item{
        {ItemName: "Go Mug", Amount: 12.49},
        {ItemName: "Go Keychain", Amount: 6.95},
        {ItemName: "Go Tshirt", Amount: 17.99},
    }
)

func main() {
    db, err := gorm.Open("sqlite3", "/tmp/gorm.db")
    db.LogMode(true)
    if err != nil {
        log.Panic(err)
    }
    defer db.Close()

    // Migrate the schema
    db.AutoMigrate(&OrderItem{}, &Order{}, &Item{})

    // Create Items
    for index := range items {
        db.Create(&items[index])
    }
    order := Order{Status: "pending"}
    db.Create(&order)
    item1 := OrderItem{OrderID: order.ID, ItemID: items[0].ID, Quantity: 1}
    item2 := OrderItem{OrderID: order.ID, ItemID: items[1].ID, Quantity: 4}
    db.Create(&item1)
    db.Create(&item2)

    // Query with joins
    rows, err := db.Table("orders").Where("orders.id = ? and status = ?", order.ID, "pending").
        Joins("Join order_items on order_items.order_id = orders.id").
        Joins("Join items on items.id = order_items.id").
        Select("orders.id, orders.status, order_items.order_id, order_items.item_id, order_items.quantity" +
            ", items.item_name, items.amount").Rows()
    if err != nil {
        log.Panic(err)
    }

    defer rows.Close()
    // Values to load into
    newOrder := &Order{}
    newOrder.OrderItems = make([]OrderItem, 0)

    for rows.Next() {
        orderItem := OrderItem{}
        item := Item{}
        err = rows.Scan(&newOrder.ID, &newOrder.Status, &orderItem.OrderID, &orderItem.ItemID, &orderItem.Quantity, &item.ItemName, &item.Amount)
        if err != nil {
            log.Panic(err)
        }
        orderItem.Item = item
        newOrder.OrderItems = append(newOrder.OrderItems, orderItem)
    }
    log.Print(pretty.Sprint(newOrder))
}

Output:

/tmp/main.go.go:55) 
[2018-06-18 18:33:59]  [0.74ms]  INSERT INTO "items" ("created_at","updated_at","deleted_at","item_name","amount") VALUES ('2018-06-18 18:33:59','2018-06-18 18:33:59',NULL,'Go Mug','12.49')  
[1 rows affected or returned ] 

(/tmp/main.go.go:55) 
[2018-06-18 18:33:59]  [0.50ms]  INSERT INTO "items" ("created_at","updated_at","deleted_at","item_name","amount") VALUES ('2018-06-18 18:33:59','2018-06-18 18:33:59',NULL,'Go Keychain','6.95')  
[1 rows affected or returned ] 

(/tmp/main.go.go:55) 
[2018-06-18 18:33:59]  [0.65ms]  INSERT INTO "items" ("created_at","updated_at","deleted_at","item_name","amount") VALUES ('2018-06-18 18:33:59','2018-06-18 18:33:59',NULL,'Go Tshirt','17.99')  
[1 rows affected or returned ] 

(/tmp/main.go.go:58) 
[2018-06-18 18:33:59]  [0.71ms]  INSERT INTO "orders" ("created_at","updated_at","deleted_at","status") VALUES ('2018-06-18 18:33:59','2018-06-18 18:33:59',NULL,'pending')  
[1 rows affected or returned ] 

(/tmp/main.go.go:61) 
[2018-06-18 18:33:59]  [0.62ms]  INSERT INTO "order_items" ("created_at","updated_at","deleted_at","order_id","item_id","quantity") VALUES ('2018-06-18 18:33:59','2018-06-18 18:33:59',NULL,'49','145','1')  
[1 rows affected or returned ] 

(/tmp/main.go.go:62) 
[2018-06-18 18:33:59]  [0.45ms]  INSERT INTO "order_items" ("created_at","updated_at","deleted_at","order_id","item_id","quantity") VALUES ('2018-06-18 18:33:59','2018-06-18 18:33:59',NULL,'49','146','4')  
[1 rows affected or returned ] 

(/tmp/main.go.go:69) 
[2018-06-18 18:33:59]  [0.23ms]  SELECT orders.id, orders.status, order_items.order_id, order_items.item_id, order_items.quantity, items.item_name, items.amount FROM "orders" Join order_items on order_items.order_id = orders.id Join items on items.id = order_items.id WHERE (orders.id = '49' and status = 'pending')  
[0 rows affected or returned ] 
--- ONLY ONE QUERY WAS USED TO FILL THE STRUCT BELOW
2018/06/18 18:33:59 {Model:      {ID:        49,
              CreatedAt: 0001-01-01 00:00:00 +0000 UTC,
              UpdatedAt: 0001-01-01 00:00:00 +0000 UTC,
              DeletedAt: nil},
 Status:     "pending",
 OrderItems: [{Model:    {ID:        0,
                          CreatedAt: 0001-01-01 00:00:00 +0000 UTC,
                          UpdatedAt: 0001-01-01 00:00:00 +0000 UTC,
                          DeletedAt: nil},
               OrderID:  49,
               ItemID:   145,
               Item:     {Model:    {ID:        0,
                                     CreatedAt: 0001-01-01 00:00:00 +0000 UTC,
                                     UpdatedAt: 0001-01-01 00:00:00 +0000 UTC,
                                     DeletedAt: nil},
                          ItemName: "Go Mug",
                          Amount:   12.489999771118164},
               Quantity: 1},
              {Model:    {ID:        0,
                          CreatedAt: 0001-01-01 00:00:00 +0000 UTC,
                          UpdatedAt: 0001-01-01 00:00:00 +0000 UTC,
                          DeletedAt: nil},
               OrderID:  49,
               ItemID:   146,
               Item:     {Model:    {ID:        0,
                                     CreatedAt: 0001-01-01 00:00:00 +0000 UTC,
                                     UpdatedAt: 0001-01-01 00:00:00 +0000 UTC,
                                     DeletedAt: nil},
                          ItemName: "Go Keychain",
                          Amount:   6.949999809265137},
               Quantity: 4}]}