GORM 的 FirstOrCreate() 方法（或 Django 的 get_or_create）如何确保只创建一行？

Question

我正在考虑将 GORM 用于一个应用程序，并且正在研究 FirstOrCreate 是如何工作的，它似乎使用了两个数据库操作。考虑这个示例脚本：

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)
}

在运行执行此操作并检查日志后，我发现（除了自动迁移之外）它使用了两个查询，一个 SELECT 和一个 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 ]

据我了解，但是，

In a SQL DBMS, the select-test-insert approach is a mistake: nothing prevents another process from inserting the "missing" row between your select and insert statements.

Django 的 get_or_create() 似乎以类似的方式工作。鉴于此模型，

from django.db import models

class User(models.Model):
    name = models.CharField(max_length=255)
    age = models.PositiveIntegerField()

如果我启用数据库日志记录和运行一个get_or_create()查询，我会看到

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)

简而言之，如果我想确保只创建一条记录，似乎我应该避免使用 ORM（例如 GORM 或 Django ORM）并编写自己的查询？

我的第二个问题是如何在 GORM 中获得 Django 的 created 布尔值的等价物。我是否应该确定结果 gorm.DB 的 RowsAffected 是否为 1 以确定是否实际创建了一行？

Answer 1

您应该只在查询模型字段上添加 UNIQUE 约束，这足以使其在数据库中保持一致

对于将向模型

添加元 class 的 Django

class Meta:
    unique_together = ['name', 'age']

对于 GORM

Name string `gorm:"unique_index:idx_name_age"`
Age  uint   `gorm:"unique_index:idx_name_age"`

GORM 的 FirstOrCreate() 方法（或 Django 的 get_or_create）如何确保只创建一行？

How can GORM's FirstOrCreate() method (or Django's get_or_create) ensure that just one row is created?

django

orm

go

go-gorm