linq - ASP.NET 核心和 Entity Framework 核心 : Left (Outer) Join in Linq

Question

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

有两个表的简单情况:

人(身份证，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 Controller 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);
}

swagger 的结果是缺少第 3 个人(没有详细文本的那些)

[
  {
    "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:

感谢您到目前为止的答案和链接。

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

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:

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()

谷歌给了我那个: "LEFT OUTER JOIN PROBLEMS #4002"
以及 "Left outer join @ Stackoverflow"

现在我不确定这是否是一些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)查询只是看起来像

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，所以属性 PersonDetails不在脚手架模型类中。但是使用它是最简单的解决方案(并且有效，甚至可以快速运行)而不是现在的连接(请参阅错误报告)。

当加入方式工作一次时，仍然对更新感到高兴。

Answer 1

如果您需要做左连接 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,
                    firstname = person.Firstname,
                    lastname = person.Lastname,
                    detailText = m.DetailText
                };

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