Question

我正在尝试通过构建一个小型原型订单管理应用程序来学习Go和Gorm。数据库是MySQL。通过简单的查询，Gorm一直很出色。然而，当试图获得涉及一对多与一对一关系的组合的结果集时，Gorm似乎不足。毫无疑问，我缺乏理解实际上是在做空。我似乎无法找到任何我想要完成的在线示例。任何帮助将不胜感激。

Go Structs

// 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
}

数据库表

orders
id | status
 1 | pending

order_items
id | order_id | item_id | quantity
 1 | 1        | 1       | 1
 2 | 1        | 2       | 4

items
id | item_name   | amount
 1 | Go Mug      | 12.49
 2 | Go Keychain | 6.95
 3 | Go T-Shirt  | 17.99

当前查询

order := &Order 
if err := db.Where("id = ? and status = ?", reqOrder.id, "pending")
.First(&order).Error; err != nil {
    fmt.Printf(err.Error())
}

db.Model(&order).Association("OrderItems").Find(&order.OrderItems)

结果（gorm进行2分组查询）

order == Order {
  id: 1,
  status: pending,
  OrderItems[]: {
    {
      ID: 1,
      OrderID: 1,
      ItemID: 1,
      Item: nil,
      Quantity: 1,
    },
    {
      ID: 2,
      OrderID: 1,
      ItemID: 2,
      Item: nil,
      Quantity: 4,
    }
 }

替代查询

order := &Order
db.Where("id = ? and status = ?", reqOrder.id, "cart")
.Preload("OrderItems").Preload("OrderItems.Item").First(&order)

结果（gorm进行3分组查询）

order == Order {
  id: 1,
  status: pending,
  OrderItems[]: {
    {
      ID: 1,
      OrderID: 1,
      ItemID: 1,
      Item: {
        ID: 1,
        ItemName: Go Mug,
        Amount: 12.49,
      }
      Quantity: 1,
    },
    {
      ID: 2,
      OrderID: 1,
      ItemID: 2,
      Item: {
        ID: 2,
        ItemName: Go Keychain,
        Amount: 6.95,
      },
      Quantity: 4,
    }
 }

理想的结果

上面的“替代查询”产生理想的查询结果。但是，Gorm会进行3次单独的数据库查询。理想情况下，使用1（或2）个数据库查询可以完成相同的结果。

这可以通过几个连接在MySQL中完成。 Gorm允许连接。但是，我希望利用Gorm的一些关系魔法。

非常感谢！

Answer 1

如此issue中所述，gorm不是为了使用连接来预加载其他结构值而设计的。如果您想继续使用gorm并且能够使用连接来加载值，则必须使用gorm中公开的SQL Builder，并编写一些代码来扫描所需的值。

如果有许多表需要考虑，这将会变得很麻烦。如果xorm可用作选项，则它们支持加载struct值。在查找项目符号here下描述。

注意：我没有扫描所有字段，只是足以得到重点。

示例：

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)) }

<强>输出：

/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}]}

Answer 2

从上面的答案中，要获取订单列表，它将是：

 .....

    orders := make([]Order, 0)

    for rows.Next() {
        order := Order{}
        orderItem := OrderItem{}

        item := Item{}
        err = rows.Scan(&order.ID, &order.Status, &orderItem.OrderID, &orderItem.ItemID, &orderItem.Quantity, &item.ItemName, &item.Amount)
        if err != nil {
            log.Panic(err)
        }

        orderItem.Item = item

        order.OrderItems = append(order.OrderItems, orderItem)

        orders = append(orders, order)
    }

    log.Print(pretty.Sprint(orders))
}

Golang Gorm一对多有一个

2 个答案: