ASP.NET Core&EntityFramework核心：左(外)加入Linq

分类：.NET Core作者：佚名
我正在尝试使用ASP.NET Core和EntityFramework Core在Linq中使用左连接.

两个表的简单情况：

人(id,名字,姓氏)

PersonDetails(id,PersonId,DetailText)

我尝试查询的数据是Person.id,Person.firstname,Person.lastname和PersonDetails.DetailText.
有些人没有DetailText,所以想要的结果是NULL.

在sql中它工作正常

SELECT p.id,p.Firstname,p.Lastname,d.DetailText FROM Person p 
LEFT JOIN PersonDetails d on d.id = p.Id 
ORDER BY p.id ASC

结果如预期：

# | id | firstname | lastname | detailtext
1 | 1  | First1    | Last1    | details1
2 | 2  | First2    | Last2    | details2
3 | 3  | First3    | Last3    | NULL

在我的Web API控制器中我查询：

[HttpGet]
public IActionResult Get()
{
    var result = from person in _dbContext.Person
                    join detail in _dbContext.PersonDetails on person.Id equals detail.PersonId
                    select new
                    {
                        id = person.Id,firstname = person.Firstname,lastname = person.Lastname,detailText = detail.DetailText
                    };
   return Ok(result);
}

招摇的结果是缺少人3(没有详细文本的人)

[
  {
    "id": 1,"firstname": "First1","lastname": "Last1","detailText": "details1"
  },{
    "id": 2,"firstname": "First2","lastname": "Last2","detailText": "details2"
  }
]

我在Linq做错了什么？

更新1：

感谢您的答案和链接到目前为止.

我使用into和.DefaultIfEmpty()复制并粘贴下面的代码,经过一些进一步的阅读后,我明白这应该有效.

不幸的是,事实并非如此.

首先,代码开始抛出异常,但仍然返回前两个结果(缺少NULL).从输出窗口复制粘贴：

System.NullReferenceException: Object reference not set to an instance of an object.
   at lambda_method(Closure,TransparentIdentifier`2 )
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer,IEnumerable values,JsonArrayContract contract,JsonProperty member,JsonContainerContract collectionContract,JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter,Object value,Type objectType)
   at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter,Type objectType)
   at Microsoft.AspNetCore.Mvc.Formatters.JsonOutputFormatter.WriteObject(TextWriter writer,Object value)
   at Microsoft.AspNetCore.Mvc.Formatters.JsonOutputFormatter.d__9.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__32.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__31.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__29.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__23.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__18.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Builder.RouterMiddleware.d__4.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.ApplicationInsights.AspNetCore.ExceptionTrackingMiddleware.d__4.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.ApplicationInsights.AspNetCore.RequestTrackingMiddleware.d__4.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Server.IISIntegration.IISMiddleware.d__8.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Hosting.Internal.RequestServicesContainerMiddleware.d__3.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Server.Kestrel.Internal.Http.Frame`1.d__2.MoveNext()
Microsoft.AspNetCore.Server.Kestrel:Error: Connection id "0HKVGPV90QGE0": An unhandled exception was thrown by the application.

System.NullReferenceException: Object reference not set to an instance of an object.
   at lambda_method(Closure,Object value)
   at Microsoft.AspNetCore.Mvc.Formatters.JsonOutputFormatter.d__9.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__32.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__31.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__29.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__23.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__18.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Builder.RouterMiddleware.d__4.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.ApplicationInsights.AspNetCore.ExceptionTrackingMiddleware.d__4.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.ApplicationInsights.AspNetCore.RequestTrackingMiddleware.d__4.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Server.IISIntegration.IISMiddleware.d__8.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Hosting.Internal.RequestServicesContainerMiddleware.d__3.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Server.Kestrel.Internal.Http.Frame`1.d__2.MoveNext()

谷歌给了我一个：“LEFT OUTER JOIN PROBLEMS #4002”
以及“Left outer join @ Stackoverflow”

现在我不确定这是否仍然存在或应该已经修复的错误.我正在使用EntityFramework Core RC2.

解决方案1：导航属性

正如Gert Arnold在评论中指出的那样：使用导航属性

这意味着(工作)查询看起来像

var result = from person in _dbContext.Person
             select new
             {
                id = person.Id,detailText = person.PersonDetails.Select(d => d.DetailText).SingleOrDefault()
            };

return Ok(result);

在我的PersonExampleDB中,我没有正确设置外键,因此属性PersonDetails不在scaffolded模型类中.但是使用它是最简单的解决方案(并且工作甚至可以快速工作)而不是现在的连接(参见错误报告).

当加入方式有效时,仍然对更新感到满意.

解决方法

如果你需要执行Left连接,那么你必须使用into和DefaultIfEmpty(),如下所示. 
var result = from person in _dbContext.Person
             join detail in _dbContext.PersonDetails on person.Id equals detail.PersonId into Details
             from m in Details.DefaultIfEmpty()
               select new
                {
                    id = person.Id,detailText = m.DetailText
                };

您可以了解更多信息：Left Outer Join in LINQ to Entities

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