Golang Gorm 一对多与 has-one

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问题内容

我正在尝试通过构建一个小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，当试图获得一个包含一对多和一对一关系组合的结果集时，Gorm 似乎达不到要求。毫无疑问，实际上是我缺乏了解。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。

去结构

// 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 db 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 db 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”会产生理想的查询结果。但是，Gorm 为此进行了 3 个单独的数据库查询。理想情况下，使用 1 个（或 2 个）数据库查询可以获得相同的结果。

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！

正确答案

如本期所述，gorm 并非旨在使用连接来预加载其他结构值。如果您想继续使用 gorm 并有能力使用连接来加载值，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，这将变得很麻烦。如果[xorm](https://github.com/go- xorm/xorm)作为选项可用，它们支持加载结构值。[在此处](https://github.com/go-xorm/xorm#quick- start)的查找项目符号下进行了描述。

注意：我没有扫描所有字段，仅足以理解要点。

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))
}

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}]}

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