ASP.NET 核心 &EntityFramework 核心:Linq 中的左(外)加入 [英] ASP.NET Core & EntityFramework Core: Left (Outer) Join in Linq

问题描述

我正在尝试使用 ASP.NET Core 和 EntityFramework Core 在 Linq 中进行左连接.

I am trying to get a left join working in Linq using ASP.NET Core and EntityFramework Core.

有两个表的简单情况:

人(id，名字，姓氏)

Person (id, firstname, lastname)

PersonDetails(id、PersonId、DetailText)

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.

The data I try to query is Person.id, Person.firstname, Person.lastname and PersonDetails.DetailText. Some persons do not have a DetailText so the wanted result is NULL.

在 SQL 中它工作正常

In SQL it works fine

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

结果符合预期:

# | 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:

inside my Web API controller i query:

[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);
}

swagger 的结果是缺少第 3 个人(没有详细文本的那些)

The results in swagger are missing Person 3 (those without detail text)

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

我在 Linq 中做错了什么?

What am I doing wrong in Linq?

感谢您到目前为止的答案和链接.

Thank you for the answers and the links so far.

我使用 into 和 .DefaultIfEmpty() 复制并粘贴了下面的代码，经过进一步阅读后，我明白这应该可行.

I copied and pasted the code(s) below using into and .DefaultIfEmpty() and after some further readings I understand that this should work.

不幸的是它没有.

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:

First there the code starts throwing exceptions but still returns with the first two results (with the NULLs missing). Copy Paste from the output window:

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, Object value, 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 , 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, Object value, 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()

Google 给了我一个:LEFT OUTER JOIN PROBLEMS #4002"以及左外连接@Stackoverflow"

Google gave me that one: "LEFT OUTER JOIN PROBLEMS #4002" as well as "Left outer join @ Stackoverflow"

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

Now I am not sure if that is some bug that either still exists or should have been fixed already. I am using EntityFramework Core RC2.

正如 Gert Arnold 在评论中指出的那样:使用导航属性

As Gert Arnold pointed out in the comments: use the navigation properties

这意味着(工作)查询看起来像

This means the (working) query simply looks like

var result = from person in _dbContext.Person
             select new
             {
                id = person.Id,
                firstname = person.Firstname,
                lastname = person.Lastname,
                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 不在脚手架模型类中.但是现在使用它是最简单的解决方案(并且有效，甚至可以快速运行)而不是连接(请参阅错误报告).

In my PersonExampleDB I hadn't correctly set the foreign key so the property PersonDetails wasn't in the scaffolded model class. But using this is the simplest solution (and works and even works fast) instead of the join for now (see the bug reports).

当加入方式一次起作用时，仍然对更新感到高兴.

Still happy about updates when the join way works one time.

推荐答案

如果你需要做左连接，那么你必须使用 into 和 DefaultIfEmpty()如下图.

If you need to do the Left joins then you have to use into and DefaultIfEmpty() as shown below.

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,
                    firstname = person.Firstname,
                    lastname = person.Lastname,
                    detailText = m.DetailText
                };

您可以了解更多信息:LINQ to Entities 中的左外连接

You can learn more about it : Left Outer Join in LINQ to Entities

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