如何使用Include方法（无Join方法）将具有多个左联接的SQL转换为Entity Framework LINQ语句？

Question

我正在尝试将此SQL语句转换为使用Include方法（无Join Methods）的相应EF6 LINQ语句，而我无法正确获得LINQ语句。 使用LinqPad。 感谢任何帮助。

SQL：

SELECT c.CustomerName, o.Description, co.CountryName, ci.CityName
FROM Customer c
LEFT JOIN [Order] o ON c.CustId = o.CustId
LEFT JOIN Country co ON co.CountryId = o.CountryId
LEFT JOIN City ci ON ci.CityId = o.CityId

我的尝试：

сustomers.Include(c => c.Orders)
         .Select(c => new { c.CustomerName, orders = c.Orders })
         .AsEnumerable()
         .Select(c => new { c.CustomerName, c.orders ... ? })
         .Dump();

DDL：

订单表具有3个对应表的外键。

CREATE TABLE [dbo].[Customer](
    [CustId] [int] NOT NULL,
    [CustomerName] [varchar](50) NOT NULL,
 CONSTRAINT [PK_Customer] PRIMARY KEY CLUSTERED 
(   [CustId] ASC )

CREATE TABLE [dbo].[Order](
    [OrderId] [int] NOT NULL,
    [CustId] [int] NOT NULL,
    [Description] [varchar](50) NOT NULL,
    [CountryId] [int] NOT NULL,
    [CityId] [int] NOT NULL,
 CONSTRAINT [PK_Order] PRIMARY KEY CLUSTERED 
(   [OrderId] ASC 

CREATE TABLE [dbo].[Country](
    [CountryId] [int] NOT NULL,
    [CountryName] [varchar](50) NOT NULL,
 CONSTRAINT [PK_Country] PRIMARY KEY CLUSTERED 
(   [CountryId] ASC ) 

CREATE TABLE [dbo].[City](
    [CityId] [int] NOT NULL,
    [CityName] [varchar](50) NOT NULL,
 CONSTRAINT [PK_City] PRIMARY KEY CLUSTERED 
(   [CityId] ASC )

Answer 1

您基本上有2个选择：

• 明确地编写联接：

   var q = from c in db.Customer join o in db.Order on c.CustId = o.CustId into oj // join from o in oj.DefaultIfEmpty() // left join join co in db.Country on o.CountryId equals co.CountryId into coj from co in coj join ci in db.City on o.CityId = ci.CityId into cij from ci in cij select new { c.CustomerName, o.Description, co.CountryName, ci.CityName }; 

• 或定义导航属性，然后让EF为您编写这2个联接：

   var q = from c in db.Customer // .Include(c => c.Orders) ? from o in c.Orders // .DefaultIfEmpty() ? select new { c.CustomerName, o.Description, o.Country.CountryName, // those two are the (navigation) properties on the Order class o.City.CityName }; public class Order { public Customer Customer { get; set; } public City City { get; set; } // public Country Country { get; set; } } internal class OrderConfiguration : EntityTypeConfiguration<Order> { public OrderConfiguration() { // EF fluent API goes here } } 

要遵循评论中的建议：

var q = from o in db.Orders
        select new
        {
            o.Description,
            o.Customer.CustomerName,
            o.City.CityName,
            o.City.Country.CountryName
        };

也可以看看：

• 在代码优先中将配置移动到单独的类
• EF-流利的API