linq to entites 左外连接

Question

我正在努力 linq 到实体左外连接。我有两个实体（表）：

Listings 
{
    ListingID,
    MakeID (nullable)
}

Makes
{
    MakeID,
    Description
}

我想在 LINQ 中编写如下内容：

select listings.listingID
,listings.makeid
, IsNull(makes.Description, 'NA')
from listings
left outer join makes
on listings.makeid = makes.makeid

Answer 1

以下是实现左连接的解决方案。就其他资源而言，我真的建议尝试 linq pad： http://www.linqpad.net/ 它是 Linq 的一个很棒的学习工具。

// Listing class/container/table
public class Listing
{
    public string ListingID {get;set;}
    public Int32? MakeID {get;set;}
}

// Make class/container/table
public class Make
{
    public Int32 MakeID {get;set;}
    public string Description {get;set;}
}

public class Main
{
    public static void LinqMain()
    {
        // Populate the listing table with data
        List<Listing> listings = new List<Listing>()
        {
            new Listing() { ListingID = "Test 1", MakeID = 1 },
            new Listing() { ListingID = "Test 2", MakeID = 1 },
            new Listing() { ListingID = "No Make", MakeID = null },
            new Listing() { ListingID = "Test 3", MakeID = 3 },
            new Listing() { ListingID = "Another Makeless", MakeID = null }
        };

        // Populate the makes table with data
        List<Make> makes = new List<Make>()
        {
            new Make() { MakeID = 1, Description = "Make 1"},
            new Make() { MakeID = 2, Description = "Make 2"},
            new Make() { MakeID = 3, Description = "Make 3"},
            new Make() { MakeID = 4, Description = "Make 4"}
        };

        // Return the left join on Make Id
        var result = from l in listings

                     // These two lines are the left join. 
                     join leftm in makes on l.MakeID equals leftm.MakeID into leftm
                     from m in leftm.DefaultIfEmpty()

                     // To ensure the select does not get bogged down with too much logic use the let syntax
                     let description = m == null ? "NA" : m.Description

                     select new { l.ListingID, l.MakeID, description };


    }

结果变量将包含：

1. { ListingID = "Test 1", MakeID = 1, description = "Make 1" }
2. { ListingID = "Test 2", MakeID = 1, description = "Make 1" }
3. { ListingID = "No Make ", MakeID = null, 描述 = "NA" }
4. { ListingID = "测试 3", MakeID = 3, 描述 = "Make 3" }
5. { ListingID = "另一个 Makeless", MakeID = null, 描述 = "NA" }

Answer 2

任何告诉您使用 .DefaultIfEmpty() 作为 LINQ to Entities 中外部联接的一部分的人实际上都没有尝试过！根本不起作用 - 至少在 .NET 3.5 SP1 上是这样。

此博主告诉您如何实际上应该这样做。本质上，.NET 默认情况下在 LINQ to Entities 中执行外部联接，因此您应该省略 .DefaultIfEmpty()。对于多个外连接，您必须嵌套查询组以保持其上下文清晰。

Answer 3

http://oddiandeveloper.blogspot。 com/2008/12/testable-left-outer-join-in-linq-to.html

这应该有帮助，这是我不久前发表的一篇博客文章，应该仍然相关，并且也可能有助于可测试性。

生成实体模型时还要确保外键就位，这将有助于设置依赖项。

Answer 4

不是在开发机器前检查，但也许是这样的？

var x = from l in listings
    join m in makes on l.makeid equals m.makeid into g
    from ma in g.DefaultIfEmpty()
    select new 
    {
        l.listingID, 
        l.makeid, 
        (ma.Description == null ? "NA" : ma.Description)
    };

如果您遇到任何问题，请告诉我，我会检查我的工作电脑。