EntityFramework的天坑

先看以下代码：

        public void Index()
        {
            //新增一条数据
            _testDbContext.Add(new User { UserName = "admin_test" });
            //从数据库查询，并修改UserName字段的值，但不显式提交修改
            var model = _testDbContext.Users.FirstOrDefault(m => m.Id == 1);
            model.UserName = "我的测试2";
            //保存
            _testDbContext.SaveChanges();
        }

按照常理来说，从数据库查询到的数据，虽然对其字段做了修改，但没显式提交，应该不会保存。但实际是保存了。
那么为何会这样？我们先把sql语句打印出来，看下都执行了什么语句。要想打印sql，需要做一些配置。
在Startup类中定义一个字段：

        public static readonly ILoggerFactory MyLoggerFactory
        = LoggerFactory.Create(builder =>
        {
            #if DEBUG
                builder.AddConsole();
            #endif
        });

然后修改ConfigureServices方法，添加UseLoggerFactory：

            var connection = Configuration.GetConnectionString("MysqlConnection");
            services.AddDbContext<TestDbContext>(options => options
                        .UseMySql(connection, ServerVersion.AutoDetect(connection))
                        .UseLoggerFactory(MyLoggerFactory)//输出sql日志
            );

主要是.UseLoggerFactory(MyLoggerFactory) 这一句，然后运行程序，查看打印结果：

info: Microsoft.Hosting.Lifetime[0]
      Now listening on: http://localhost:5001
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: C:UsersBenDesktopEFTestEFTest
info: Microsoft.EntityFrameworkCore.Infrastructure[10403]
      Entity Framework Core 5.0.10 initialized 'TestDbContext' using provider 'Pomelo.EntityFrameworkCore.MySql' with options: ServerVersion 5.7.22-mysql
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (45ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
      SELECT `u`.`Id`, `u`.`UserName`
      FROM `Users` AS `u`
      WHERE `u`.`Id` = 1
      LIMIT 1
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (40ms) [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 4000)], CommandType='Text', CommandTimeout='30']
      UPDATE `Users` SET `UserName` = @p0
      WHERE `Id` = @p1;
      SELECT ROW_COUNT();
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (23ms) [Parameters=[@p0='?' (Size = 4000)], CommandType='Text', CommandTimeout='30']
      INSERT INTO `Users` (`UserName`)
      VALUES (@p0);
      SELECT `Id`
      FROM `Users`
      WHERE ROW_COUNT() = 1 AND `Id` = LAST_INSERT_ID();

上面的打印结果，显示：
1.查询id==1的数据
2.额外执行了一条update语句
3.执行了一条insert语句
由此可以得出结论：被dbcontext追踪的数据，在被修改后，即使不显式调用update方法，ef也会自动保存数据。

代码不变，再运行一次，看结果：

info: Microsoft.Hosting.Lifetime[0]
      Now listening on: http://localhost:5001
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: C:UsersBenDesktopEFTestEFTest
info: Microsoft.EntityFrameworkCore.Infrastructure[10403]
      Entity Framework Core 5.0.10 initialized 'TestDbContext' using provider 'Pomelo.EntityFrameworkCore.MySql' with options: ServerVersion 5.7.22-mysql
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (46ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
      SELECT `u`.`Id`, `u`.`UserName`
      FROM `Users` AS `u`
      WHERE `u`.`Id` = 1
      LIMIT 1
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (24ms) [Parameters=[@p0='?' (Size = 4000)], CommandType='Text', CommandTimeout='30']
      INSERT INTO `Users` (`UserName`)
      VALUES (@p0);
      SELECT `Id`
      FROM `Users`
      WHERE ROW_COUNT() = 1 AND `Id` = LAST_INSERT_ID();

怎么没有修改语句了？原因是：model.UserName虽然赋值了，但是实际值和数据库中的值相同，没有发生变化，所以ef不会修改。
个人建议：在不了解ef原理的情况下不要这么写：

        public void Index()
        {
            //新增一条数据
            _testDbContext.Add(new User { UserName = "admin_test" });
            //从数据库查询，并修改UserName字段的值，但不显式提交修改
            var model = _testDbContext.Users.FirstOrDefault(m => m.Id == 1);
            model.UserName = "我的测试2";
            //保存
            _testDbContext.SaveChanges();
        }

如果想避免这种情况，其实也是可以的，只要在查询的时候，添加AsNoTracking就行了：

        public void Index()
        {
            //新增一条数据
            _testDbContext.Add(new User { UserName = "admin_test" });
            //从数据库查询，并修改UserName字段的值，但不显式提交修改
            var model = _testDbContext.Users.AsNoTracking().FirstOrDefault(m => m.Id == 1);
            model.UserName = "我的测试22";
            //保存
            _testDbContext.SaveChanges();
        }

AsNoTracking的作用是：告诉EntityFramework不对查询结果进行追踪。

测试代码：https://gitee.com/subendong/EFTest

有篇文章写的很好，可以仔细阅读：https://www.cnblogs.com/sheng-jie/p/7416302.html