[英]How can GORM's FirstOrCreate() method (or Django's get_or_create) ensure that just one row is created?
我正在考慮將 GORM 用於應用程序,並正在研究FirstOrCreate
工作原理,它似乎使用了兩個數據庫操作。 考慮這個示例腳本:
package main
import (
"github.com/jinzhu/gorm"
_ "github.com/jinzhu/gorm/dialects/sqlite"
"github.com/sirupsen/logrus"
)
type User struct {
gorm.Model
Name string
Age uint
}
func main() {
db, err := gorm.Open("sqlite3", "examplegorm.db")
if err != nil {
logrus.Fatalf("open db: %v", err)
}
defer db.Close()
db.LogMode(true)
db.AutoMigrate(&User{})
var user User
db.Where(User{Name: "non_existing"}).Attrs(User{Age: 20}).FirstOrCreate(&user)
}
運行它並檢查日志后,我看到(除了自動遷移)它使用兩個查詢,一個SELECT
和一個INSERT
:
kurt@Kurts-MacBook-Pro-13 ~/D/Scratch> go run gorm_example.go
(/Users/kurt/Documents/Scratch/gorm_example.go:23)
[2020-01-05 09:09:10] [1.03ms] CREATE TABLE "users" ("id" integer primary key autoincrement,"created_at" datetime,"updated_at" datetime,"deleted_at" datetime,"name" varchar(255),"age" integer )
[0 rows affected or returned ]
(/Users/kurt/Documents/Scratch/gorm_example.go:23)
[2020-01-05 09:09:10] [0.86ms] CREATE INDEX idx_users_deleted_at ON "users"(deleted_at)
[0 rows affected or returned ]
(/Users/kurt/Documents/Scratch/gorm_example.go:26)
[2020-01-05 09:09:10] [0.28ms] SELECT * FROM "users" WHERE "users"."deleted_at" IS NULL AND (("users"."name" = 'non_existing')) ORDER BY "users"."id" ASC LIMIT 1
[0 rows affected or returned ]
(/Users/kurt/Documents/Scratch/gorm_example.go:26)
[2020-01-05 09:09:10] [0.31ms] INSERT INTO "users" ("created_at","updated_at","deleted_at","name","age") VALUES ('2020-01-05 09:09:10','2020-01-05 09:09:10',NULL,'non_existing',20)
[1 rows affected or returned ]
然而,據我了解https://stackoverflow.com/a/16128088/995862 ,
在 SQL DBMS 中,select-test-insert 方法是一個錯誤:沒有什么可以阻止另一個進程在您的
select
和insert
語句之間插入“缺失”的行。
似乎 Django 的get_or_create()
以類似的方式工作。 鑒於這個模型,
from django.db import models
class User(models.Model):
name = models.CharField(max_length=255)
age = models.PositiveIntegerField()
如果我啟用數據庫日志記錄並運行get_or_create()
查詢,我會看到
In [1]: from djangoapp.models import *
In [2]: User.objects.get_or_create(name="jinzhu", age=20)
(0.000) SELECT "djangoapp_user"."id", "djangoapp_user"."name", "djangoapp_user"."age" FROM "djangoapp_user" WHERE ("djangoapp_user"."age" = 20 AND "djangoapp_user"."name" = 'jinzhu') LIMIT 21; args=(20, 'jinzhu')
(0.000) BEGIN; args=None
(0.000) INSERT INTO "djangoapp_user" ("name", "age") VALUES ('jinzhu', 20); args=['jinzhu', 20]
Out[2]: (<User: User object (1)>, True)
簡而言之,如果我想確保只創建一條記錄,似乎我應該避免使用諸如 GORM 或 Django ORM 之類的 ORM 並編寫自己的查詢?
我的第二個問題是如何在 GORM 中獲得 Django created
Boolean 的等價物。 我應確定是否RowsAffected
所產生的gorm.DB
為1
來確定行是否實際創建或不?
您應該只在查詢模型字段上添加 UNIQUE 約束,這足以使其在 db 中保持一致
對於將元類添加到模型的 Django
class Meta:
unique_together = ['name', 'age']
對於 GORM
Name string `gorm:"unique_index:idx_name_age"`
Age uint `gorm:"unique_index:idx_name_age"`
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