多个左联接的LINQ方法语法
本文关键字:LINQ 方法 语法 | 更新日期: 2023-09-27 18:20:12
需要将三个表连接在一起。
Table [Package]
ID (int)
ContainerID (int)
Code (string)
Code2 (string)
Table [UserHasPackages]
UserID (Comes from Identity tables) (string)
PackageID (int)
Table [Container]
ID (int)
Name (string)
Description (string)
进入表示我想在视图中显示的对象的视图模型:
public class CustomViewModel
{
public int ID { get; set; }
public string Name { get; set; } // Container.Name
public string Code { get; set; } // Package.Code
public string Code2 { get; set; } // Package.Code2
}
但我在加入时遇到了问题:
List<CustomViewModel> list = new List<CustomViewModel>();
list = context.Packages.Join(
context.Containers,
p => p.ContainerID,
c => c.ID,
(p, c) => new { p, c})
.Join( //error is here
context.UserHasPackages,
a => a.p.ID,
b => b.ApplicationUserId,
(a, b) => new { a, b })
.Select(f => new CustomViewModel
{
ID = f.p.ID,
Name = f.c.Name,
Code = f.p.Code,
Code2 = f.p.Code2
}).ToList();
Type arguments for method cannot be inherited from the usage. Try specifying the type arguments explicitly.
有没有其他更合适的方法来进行两个连接(如所述)?
解决方案
基于方法的语法在这里是不可行的,必须使用查询语法:
var query = (from package in context.Packages
join container in context.Containers on package.ContainerID equals container.ID
join userHasPackage in context.UserHasPackages on package.ID equals userHasPackage.PackageID
where userHasPackage.UserID == "SomeUser"
select new CustomViewModel
{
ID = package.ID,
Name = container.Name,
Code = package.Code,
Code2 = package.Code2
}).ToList();
我假设您想要加入UserHasPackages表,因为您想要筛选特定用户的结果(我只是输入了一个"SomeUser",因为我不确定"UserHasPackages.ApplicationUserId"来自哪里),因为它不包括在视图模型中。
我认为以下内容应该有效:
var list = context.Packages
.Join(context.Containers, p => p.ContainerID, c => c.ID, (p, c) => new { p, c })
.Join(context.UserHasPackages, pc => pc.p.ID, u => u.PackageID, (pc, u) => new { pc.p, pc.c, u })
.Where(pcu => pcu.u.UserID == "SomeUser")
.Select(pcu => new
{
pcu.p.ID,
pcu.c.Name,
pcu.p.Code,
pcu.p.Code2
});
您也可以使用查询语法:
var query = from package in context.Packages
join container in context.Containers on package.ContainerID equals container.ID
join userHasPackage in context.UserHasPackages on package.ID equals userHasPackage.PackageID
where userHasPackage.UserID == "SomeUser"
select new
{
package.ID,
container.Name,
package.Code,
package.Code2
};