我正在尝试通过构建一个原型订单管理应用程序来学习Go和Gorm。该数据库是MySQL。通过简单的查询，Gorm表现出色。但是，当尝试获取涉及具有一对一关系的一对多组合的结果集时，Gorm似乎不足。毫无疑问，实际上是我缺乏理解。我似乎找不到任何我想完成的在线示例。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个数据库查询）
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个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”产生理想的查询结果。但是，Gorm会进行3个单独的数据库查询。理想情况下，使用1（或2）个数据库查询可以实现相同的结果。
可以通过几个联接在MySQL中完成。Gorm允许加入。但是，我希望利用Gorm的一些关系魔术。
谢谢一群！
如本期所述，gorm并非设计为使用联接来预加载其他结构值。如果您想继续使用gorm并能够使用联接加载值，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。
如果必须考虑许多表，这将变得很麻烦。如果可以选择使用xorm，则它们支持加载结构值。在此处找到要点下方描述。
注意：我并没有扫描所有字段，仅能说明问题。
范例 ：
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}]}
