实体框架似乎不必要地加入同一个表两次

Question

更新这可能已经修复：http://entityframework.codeplex.com/workitem/486

...

针对我的实体的相当简单的 LINQ 语句会导致不必要的复杂 SQL。稍后会详细介绍，这是设置：

表格

出版

• PublicationId (pk)
• TopicId（转至主题表）
• ReceiptCount（针对查询性能进行了非规范化处理）
• 插入日期

收据

• ReceiptId (pk)
• PublicationId（参考上表）
• 插入日期

LINQ

var query = from r in context.Receipts.Include("Publication")
            where r.DateInserted < lagDate
            && r.ReceiptId > request.AfterReceiptId
            && r.Publication.TopicId == topicEntity.TopicId
            && r.Publication.ReceiptCount > 1
            select r;

SQL

exec sp_executesql N'SELECT TOP (25) 
[Project1].[ReceiptId] AS [ReceiptId], 
[Project1].[PublicationId] AS [PublicationId], 
[Project1].[DateInserted] AS [DateInserted], 
[Project1].[DateReceived] AS [DateReceived], 
[Project1].[PublicationId1] AS [PublicationId1], 
[Project1].[PayloadId] AS [PayloadId], 
[Project1].[TopicId] AS [TopicId], 
[Project1].[BrokerType] AS [BrokerType], 
[Project1].[DateInserted1] AS [DateInserted1], 
[Project1].[DateProcessed] AS [DateProcessed], 
[Project1].[DateUpdated] AS [DateUpdated], 
[Project1].[PublicationGuid] AS [PublicationGuid], 
[Project1].[ReceiptCount] AS [ReceiptCount]
FROM ( SELECT 
    [Extent1].[ReceiptId] AS [ReceiptId], 
    [Extent1].[PublicationId] AS [PublicationId], 
    [Extent1].[DateInserted] AS [DateInserted], 
    [Extent1].[DateReceived] AS [DateReceived], 
    [Extent3].[PublicationId] AS [PublicationId1], 
    [Extent3].[PayloadId] AS [PayloadId], 
    [Extent3].[TopicId] AS [TopicId], 
    [Extent3].[BrokerType] AS [BrokerType], 
    [Extent3].[DateInserted] AS [DateInserted1], 
    [Extent3].[DateProcessed] AS [DateProcessed], 
    [Extent3].[DateUpdated] AS [DateUpdated], 
    [Extent3].[PublicationGuid] AS [PublicationGuid], 
    [Extent3].[ReceiptCount] AS [ReceiptCount]
    FROM   [dbo].[Receipt] AS [Extent1]
    INNER JOIN [dbo].[Publication] AS [Extent2] ON [Extent1].[PublicationId] = [Extent2].[PublicationId]
    LEFT OUTER JOIN [dbo].[Publication] AS [Extent3] ON [Extent1].[PublicationId] = [Extent3].[PublicationId]
    WHERE ([Extent2].[ReceiptCount] > 1) AND ([Extent1].[DateInserted] < @p__linq__0) AND ([Extent1].[ReceiptId] > @p__linq__1) AND ([Extent2].[TopicId] = @p__linq__2)
)  AS [Project1]
ORDER BY [Project1].[ReceiptId] ASC',N'@p__linq__0 datetime,@p__linq__1 int,@p__linq__2 int',@p__linq__0='2012-09-05 19:39:21:510',@p__linq__1=4458824,@p__linq__2=90

问题

出版物被加入两次：

1. 因为.Include("Publication")而左外连接
2. INNER JOIN 因为where。

如果我从 SQL 中完全删除 [Extent2]，并更改 WHERE 位以使用 [Extent3]，我会得到相同的结果。由于我没有在我的实体上使用延迟加载，所以我必须 .Include("Publication")... 有什么解决方案吗？

我使用的是 EF4，但从 NuGet 获取 EF5 以查看它是否已修复，但它产生了相同的结果（尽管我不知道如何判断我的 EDMX 是否真的在使用 EF5）。

Answer 1

但是，有一种解决方法。它可能不是最优雅的解决方案，但它完全符合您的要求；它只生成一个连接。

变化：

var query = from r in context.Receipts.Include("Publication")    
            where r.DateInserted < lagDate 
            && r.ReceiptId > request.AfterReceiptId 
            && r.Publication.TopicId == topicEntity.TopicId 
            && r.Publication.ReceiptCount > 1 
            select r; 

成为：

var query = from r in context.Receipts
            join pub in context.Publication on r.PublicationId equals pub.PublicationId
            where r.DateInserted < lagDate 
            && r.ReceiptId > request.AfterReceiptId 
            && pub.TopicId == topicEntity.TopicId 
            && pub.ReceiptCount > 1 
            select new {
                Receipt = r,
                Publication = pub
            }; 

请注意，我们已经删除了 Include 并且我们不再使用 r.Publication。？？在 where 子句中。相反，我们使用的是 pub.??

现在当你循环查询时，你会看到 r.Publication 不为空：

foreach ( var item in query)
{
    //see that item.Publication is not null
    if(item.Receipt != null && item.Receipt.Publication != null)
    {
        //do work based on a valid Publication
    }
    else
    {
        //do work based on no linked Publication
    }
}

Answer 2

可以通过使用临时变量来避免这种行为（例如，let pub = r.Publication）。

var query = from r in context.Receipts
            let pub = r.Publication // using a temp variable
            where r.DateInserted < lagDate
            && r.ReceiptId > request.AfterReceiptId
            && pub.TopicId == topicEntity.TopicId
            && pub.ReceiptCount > 1
            select new { r, pub };

Answer 3

我将通过更改以下代码来优化前人的答案，这消除了加入的需要，因此您不必知道需要加入哪些列，也不必在加入条件获得时更改 LINQ改变了。这应该是不必要的，但 MS 现在并不专注于修复他们的 sql 代码生成。

    var query = from pub in context.Publications
                from r in pub.Reciepts
                where r.DateInserted < lagDate 
                && r.ReceiptId > request.AfterReceiptId 
                && pub.TopicId == topicEntity.TopicId 
                && pub.ReceiptCount > 1 
                select new {
                       Receipt = r,
                       Publication = pub
                };