NET核心&；EntityFramework核心：Linq中的左（外）连接

我正在尝试使用ASP.NET Core和EntityFramework Core在Linq中实现左连接

有两个表的简单情况：

个人（身份证、名、姓）

个人详细信息（id、个人id、详细文本）

我尝试查询的数据是Person.id、Person.firstname、Person.lastname和PersonDetails.DetailText。 有些人没有DetailText，因此想要的结果为空

在SQL中，它工作得很好

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);
}

在swagger中的结果是失踪人员3（那些没有详细文本）

我在林克做错了什么

---

更新1： 谢谢你迄今为止的答案和链接

我使用

将下面的代码复制并粘贴到

和

.DefaultIfEmpty（）

中，在进一步阅读之后，我明白这应该是可行的

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, 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()

谷歌给了我一个： 以及

现在我不确定这是否是某个仍然存在或应该已经修复的bug。我正在使用EntityFramework核心RC2

---

解决方案1：导航属性 正如Gert Arnold在评论中指出的：使用导航属性

这意味着（工作）查询看起来就像

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。但是使用这个方法是最简单的解决方案（而且可以工作，甚至工作得很快），而不是现在的连接（参见bug报告）

---

---

当连接方式工作一次时，仍然对更新感到满意。

如果需要执行左连接，则必须使用

进入
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：
如果您不使用左连接，那么您使用的linq基本上是创建一个内部连接。对于linq中的左连接，请使用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);
}

我同意这篇文章的作者——它看起来仍然像一个bug！如果有空的联接表，则始终会收到“对象引用未设置为对象的实例”。唯一的方法是将联接表检查为null：

    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
        };

IEnumerable clubServices=
来自服务行业的
从clubs.Where中的c开始（club=>club.ClubId==s.ClubId）.DefaultIfEmpty（）
来自clubs.Where中的t（tenant=>tenant.TenantId==c.TenantId）.DefaultIfEmpty（）
选择新型号。服务
{
ServiceId=s.ServiceId.ToString（），
ClubId=c==null？（int？）null:c.ClubId，
ClubName=c==null？null:c.名称，
HastinMetable=s.HastinMetable，
MultipleCount=s.MultipleCount，
Name=s.Name，
Tags=s.Tags.Split（“；”，StringSplitOptions.RemoveEmptyEntries），
TenantId=t==null？（int？）null:t.TenantId，
租户名称=t==null？null:t.名称
};

我无法选中“
detailText=person.PersonDetails.Select（d=>d.detailText.SingleOrDefault（）
”，因为我的联接表位于不同的数据库中。
为什么要联接？使用导航属性。这是我第一次使用linq。我对一切有用的东西都开放：-）@GertArnold谢谢你的链接。我读过了。因为我使用了数据库优先的方法并构建了模型类，所以我具有导航属性。但我不知道这如何帮助我检索丢失的条目。介意再给我一个提示吗？那么你有一个财产人。个人信息？新的错误表明你可能在
选择
之前的某个地方有一个
ToList
（或
可计算的
）。into在EF Core 3中不起作用。EF Core 3不允许客户端执行。您好。“网络链接没有任何结果。”ValterEkholm现在提交了一份官方文件。
    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
        };