带有gorm的自动预加载无法正常工作

Question

我正在尝试自动预加载模型，但是这样做很困难。

这些是我正在使用的模型：

package domain

type User struct {
    gorm.Model
    Name string
    Car Car `gorm:"auto_preload"`
    Account Account `gorm:"auto_preload"`
}

type Car struct {
    gorm.Model
    Type int
    UserID uint
}

type Account struct {
    gorm.Model
    Powerlevel int
    UserID uint
}

这是我正在执行的用于测试自动预加载功能的代码：

func main() {

    db, err := gorm.Open("sqlite3", "./data.db")

    if err != nil {
        println(err.Error())
    }

    //Migrate the tables
    db.AutoMigrate(&domain.User{})
    db.AutoMigrate(&domain.Car{})
    db.AutoMigrate(&domain.Account{})

    // Initialize our models
    var testUser domain.User
    var car = domain.Car{Type: 1994}
    var account = domain.Account{Powerlevel: 9001}

    testUser.Name = "Jim Bob"
    testUser.Car = car
    testUser.Account = account

    // Save the user
    db.Create(&testUser)

    // Get a new user
    var newUser domain.User
    db.First(&newUser)

    println(newUser.Name)
    println(newUser.Car.Type)
    println(newUser.Account.Powerlevel)

    // Explicitly preload

    db.Preload("Car").First(&newUser)
    db.Preload("Account").First(&newUser)


    println(newUser.Name)
    println(newUser.Car.Type)
    println(newUser.Account.Powerlevel)
}

这两个打印的输出是：

Jim Bob
0
0
Jim Bob
1994
9001

当我可以手动很好地预加载模型时，为什么通过db.First（…）获取模型不会自动预加载？

Answer 1

首先，我建议阅读文档。
这里有点解释。看来您使用旧版本的可能性更大。更新的文档说明了如何使用此标记gorm:"PRELOAD"，默认情况下将其设置为true。因此，如果您想禁用字段使用gorm:"PRELOAD:false"。

之后，必须使用db.Set("gorm:auto_preload", true)才能正确获取它。如果要通过应用程序使用预加载，则可以重新分配db以具有auto_preload功能：

db = db.Set("gorm:auto_preload", true)

例：

package main

import "github.com/jinzhu/gorm"
import _ "github.com/jinzhu/gorm/dialects/sqlite"

type User struct {
        gorm.Model
        Name    string
        Car     Car
        Account Account `gorm:"PRELOAD:false"`
}

type Car struct {
        gorm.Model
        Type   int
        UserID uint
}

type Account struct {
        gorm.Model
        Powerlevel int
        UserID     uint
}

func main() {
        db, err := gorm.Open("sqlite3", "./data.db")
        db.LogMode(true)
        if err != nil {
                println(err.Error())
        }
        //Migrate the tables
        db.AutoMigrate(&User{})
        db.AutoMigrate(&Car{})
        db.AutoMigrate(&Account{})

        // Initialize our models
        var testUser User
        var car = Car{Type: 1994}
        var account = Account{Powerlevel: 9001}

        testUser.Name = "Jim Bob"
        testUser.Car = car
        testUser.Account = account

        // Save the user
        db.Create(&testUser)

        // Get a new user
        var newUser User


        // override db instance to always prefetch
        //db = db.Set("gorm:auto_preload", true)
        db.Set("gorm:auto_preload", true).First(&newUser)

        println(newUser.Name)
        println(newUser.Car.Type)
        println(newUser.Account.Powerlevel)

        // Explicitly preload

        // No need to use two seperate statements, you can just chain them :)
        db.Preload("Car").Preload("Account").First(&newUser)
        //db.Preload("Account").First(&newUser)

        println(newUser.Name)
        println(newUser.Car.Type)
        println(newUser.Account.Powerlevel)
}

输出：

(/home/william/main.go:46)
[2018-06-15 08:56:53]  [0.34ms]  INSERT INTO "users" ("created_at","updated_at","deleted_at","name") VALUES ('2018-06-15 08:56:53','2018-06-15 08:56:53',NULL,'Jim Bob')
[1 rows affected or returned ]

(/home/william/main.go:46)
[2018-06-15 08:56:53]  [0.11ms]  INSERT INTO "cars" ("created_at","updated_at","deleted_at","type","user_id") VALUES ('2018-06-15 08:56:53','2018-06-15 08:56:53',NULL,'1994','9')
[1 rows affected or returned ]

(/home/william/main.go:46)
[2018-06-15 08:56:53]  [0.10ms]  INSERT INTO "accounts" ("created_at","updated_at","deleted_at","powerlevel","user_id") VALUES ('2018-06-15 08:56:53','2018-06-15 08:56:53',NULL,'9001','9')
[1 rows affected or returned ]

(/home/william/main.go:51)
[2018-06-15 08:56:53]  [0.69ms]  SELECT * FROM "users"  WHERE "users"."deleted_at" IS NULL ORDER BY "users"."id" ASC LIMIT 1
[1 rows affected or returned ]

(/home/william/main.go:51)
[2018-06-15 08:56:53]  [1.93ms]  SELECT * FROM "cars"  WHERE "cars"."deleted_at" IS NULL AND (("user_id" IN ('1'))) ORDER BY "cars"."id" ASC 
[1 rows affected or returned ]
Jim Bob
1994 <-- notice here preload is on
0  <-- notice here preload was turned off Account Type

(/home/william/main.go:59)
[2018-06-15 08:56:53]  [0.28ms]  SELECT * FROM "users"  WHERE "users"."deleted_at" IS NULL AND "users"."id" = '1' ORDER BY "users"."id" ASC LIMIT 1
[1 rows affected or returned ]

(/home/william/main.go:59)
[2018-06-15 08:56:53]  [0.33ms]  SELECT * FROM "cars"  WHERE "cars"."deleted_at" IS NULL AND (("user_id" IN ('1'))) ORDER BY "cars"."id" ASC 
[1 rows affected or returned ]

(/home/william/main.go:60)
[2018-06-15 08:56:53]  [0.27ms]  SELECT * FROM "users"  WHERE "users"."deleted_at" IS NULL AND "users"."id" = '1' ORDER BY "users"."id" ASC LIMIT 1
[1 rows affected or returned ]

(/home/william/main.go:60)
[2018-06-15 08:56:53]  [0.46ms]  SELECT * FROM "accounts"  WHERE "accounts"."deleted_at" IS NULL AND (("user_id" IN ('1'))) ORDER BY "accounts"."id" ASC
[1 rows affected or returned ]

(/home/william/main.go:60)
[2018-06-15 08:56:53]  [0.40ms]  SELECT * FROM "cars"  WHERE "cars"."deleted_at" IS NULL AND (("user_id" IN ('1'))) ORDER BY "cars"."id" ASC 
[1 rows affected or returned ]
Jim Bob
1994
9001