ASP.NET Core 和 Entity Framework Core：Linq 中的左（外）连接答案

【问题标题】：ASP.NET Core & EntityFramework Core: Left (Outer) Join in LinqASP.NET Core 和 Entity Framework Core：Linq 中的左（外）连接
【发布时间】：2016-10-07 14:06:20
【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。

两个表的简单情况：

人物（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：


[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中的结果缺少Person 3（那些没有详细文字的）


[
  {
    "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？

更新 1：

感谢您到目前为止的答案和链接。

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，我知道这应该可行。

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()

Google 给了我那个："LEFT OUTER JOIN PROBLEMS #4002" 还有"Left outer join @ Stackoverflow"

现在我不确定这是仍然存在还是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 cmets 中指出的那样：使用导航属性

这意味着（工作的）查询看起来像


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，因此属性 PersonDetails 不在脚手架模型类中。但是使用这是最简单的解决方案（并且工作甚至工作速度很快）而不是现在的连接（请参阅错误报告）。

当加入方式一次有效时，仍然对更新感到高兴。

【问题讨论】：

你为什么加入？使用导航属性。
嗯，这是我第一次使用 linq。我对所有有效的东西都开放 :-)
@GertArnold 感谢您的链接。我读了。由于我使用了数据库优先方法并为模型类搭建了脚手架，因此我拥有导航属性。但我看不出这如何帮助我检索丢失的条目。介意再给我一个提示吗？
所以你有一个属性 Person.PersonDetails？
新错误表明您可能在select 之前的某个位置有一个ToList（或AsEnumerable）。

标签： linq asp.net-core entity-framework-core

【解决方案1】：

如果您需要进行左连接，则必须使用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

【讨论】：

【解决方案2】：

你没有做左连接，你使用的 linq 基本上是创建一个内连接。对于 linq 中的左连接，使用 into 关键字

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

【讨论】：

into 在 EF Core 3 中不起作用。EF Core 3 不允许客户端执行。

【解决方案3】：

我同意这篇文章的作者 - 它仍然看起来像一个错误！如果您有空的连接表，您总是会收到“对象引用未设置为对象的实例。”。唯一的方法是检查连接表是否为空：

    IEnumerable<Models.Service> clubServices =
        from s in services
        from c in clubs.Where(club => club.ClubId == s.ClubId).DefaultIfEmpty()
        from t in clubs.Where(tenant => tenant.TenantId == c.TenantId).DefaultIfEmpty()
        select new Models.Service
        {
            ServiceId = s.ServiceId.ToString(),
            ClubId = c == null ? (int?)null : c.ClubId,
            ClubName = c == null ? null : c.Name,
            HasTimeTable = s.HasTimeTable,
            MultipleCount = s.MultipleCount,
            Name = s.Name,
            Tags = s.Tags.Split(';', StringSplitOptions.RemoveEmptyEntries),
            TenantId = t == null ? (int?)null : t.TenantId,
            TenantName = t == null ? null : t.Name
        };

我无法检查“detailText = person.PersonDetails.Select(d => d.DetailText).SingleOrDefault()”，因为我的联接表位于不同的数据库中。

【讨论】：