带有gorm的自动预加载无法正常工作 [英] Auto Preloading with gorm not working as expected
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问题描述
我正在尝试自动预加载模型，但是这样做很困难.
I am trying to auto-preload my models but am having difficulty doing so.
这些是我正在使用的模型:
These are the models i am using :
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
}
这是我正在测试自动预加载功能的代码:
This is the code i am executing to test the auto-preloading feature:
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)
}
两张照片的输出是:
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?
Why does getting the model over db.First(...) not automatically preload when i can preload them manually perfectly fine ?
推荐答案
首先，我建议阅读，默认情况下将其设置为true.因此，如果您想禁用字段，请使用 gorm:"PRELOAD:false" .
Firstly I would recommend reading the docs.
A bit of explanation here. It appears that you are using an old version more than likely. The updated docs explain to use this tag gorm:"PRELOAD", which by default is set to true. So if you would like to disable a field use gorm:"PRELOAD:false".
此后，使用必须使用 db.Set("gorm:auto_preload"，true)才能正确获取.如果要在应用程序中使用预加载提取，则可以重新分配数据库，使其具有auto_preload功能:
Afterward, use must use db.Set("gorm:auto_preload", true) for it to fetch properly.
If you are going to use the preload fetch through the application then you can reassign db as such to have the auto_preload feature:
db = db.Set("gorm:auto_preload", true)
示例:
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
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