C# Linq到实体:一次选择多对多
我有这个域名:C# Linq到实体:一次选择多对多,c#,entity-framework,linq-to-entities,C#,Entity Framework,Linq To Entities,我有这个域名: public class User { public long Id { get; set; } public string Login { get; set; } public string Password { get; set; } } public class Application { public long Id { get; set; } public string Name { get; set; } } public
public class User
{
public long Id { get; set; }
public string Login { get; set; }
public string Password { get; set; }
}
public class Application
{
public long Id { get; set; }
public string Name { get; set; }
}
public class UserApplications
{
[ForeignKey("User")]
public long UserId { get; set; }
public User User { get; set; }
[ForeignKey("Application")]
public long ApplicationId { get; set; }
public Application Application { get; set; }
public DateTime LastConnection { get; set; }
}
我想做一个选择,返回如下内容:
List of select new
{
User = user,
Applications = applications // List of all user's applications
}
我尝试:
from u in Users
join ua in UserApplications on u.Id equals ua.UserId into userApplications
from ua in userApplications.DefaultIfEmpty()
join a in Applications on ua.ApplicationId equals a.Id into applications
select new
{
User = u,
Applications = applications
}
但这会将用户重复到每个应用程序
我知道我可以用两个select语句来实现,但我不想这样
我该怎么做呢?试试这个:
var tmp =
from u in Users
join ua in UserApplications on u.Id equals ua.UserId
join a in Applications on ua.ApplicationId equals a.Id
select new
{
User = u,
App = a
};
var res = tmp
.ToArray() // edited
.GroupBy(_ => _.User)
.Select(_ => new
{
User = _.Key,
Applications = _.Select(_ => _.App).ToArray()
});
试试这个:
var tmp =
from u in Users
join ua in UserApplications on u.Id equals ua.UserId
join a in Applications on ua.ApplicationId equals a.Id
select new
{
User = u,
App = a
};
var res = tmp
.ToArray() // edited
.GroupBy(_ => _.User)
.Select(_ => new
{
User = _.Key,
Applications = _.Select(_ => _.App).ToArray()
});
实际上,您只需对用户设置的
UserApplications
实体进行分组:
context
.UserApplications
.GroupBy(_ => _.User, _ => _.Application)
.ToList();
因为,事实上,i分组
是您所需要的(Key
是用户,分组项是他的应用程序)
任何其他改进都是趣味问题,比如投影到匿名类型:
context
.UserApplications
.GroupBy(_ => _.User, _ => _.Application)
.Select(_ => new
{
User = _.Key,
// since IGrouping<User, Application> is IEnumerable<Application>,
// we colud return a grouping directly
Applications = _
})
.ToList();
实际上,您只需对用户设置的
UserApplications
实体进行分组:
context
.UserApplications
.GroupBy(_ => _.User, _ => _.Application)
.ToList();
因为,事实上,i分组
是您所需要的(Key
是用户,分组项是他的应用程序)
任何其他改进都是趣味问题,比如投影到匿名类型:
context
.UserApplications
.GroupBy(_ => _.User, _ => _.Application)
.Select(_ => new
{
User = _.Key,
// since IGrouping<User, Application> is IEnumerable<Application>,
// we colud return a grouping directly
Applications = _
})
.ToList();
我不记得实体框架是否可以基于实体对象本身执行
groupby
(并在幕后提取它的Id
,并根据需要替换东西等等);但此代码适用于这种情况:
var q = from uapp in cntxt.UserApplications
group uapp by uapp.UserId
into g
select new { UserId = g.Key, Applications = g.Select(x => x.Application) };
如果您希望已提取用户
:
var q2 = from uapp in cntxt.UserApplications
group uapp by uapp.UserId
into g
let u = Users.First(x => x.Id == g.Key)
select new { User = u, Applications = g.Select(x => x.Application) };
假设您正在针对实体框架上下文编写查询,而不仅仅是尝试执行Linq到对象查询。我不记得实体框架是否可以基于实体对象本身执行
groupby
(并在场景后面提取它的Id
,并根据需要替换东西等等);但此代码适用于这种情况:
var q = from uapp in cntxt.UserApplications
group uapp by uapp.UserId
into g
select new { UserId = g.Key, Applications = g.Select(x => x.Application) };
如果您希望已提取用户
:
var q2 = from uapp in cntxt.UserApplications
group uapp by uapp.UserId
into g
let u = Users.First(x => x.Id == g.Key)
select new { User = u, Applications = g.Select(x => x.Application) };
假设您正在针对实体框架上下文编写查询,而不仅仅是尝试执行Linq to Objects查询。我担心,这将失败,因为
tmp
仍然是IQueryable
,因此ToArray
无法转换为SQL查询。我担心,这将失败,由于tmp
仍然是IQueryable
,因此ToArray
无法转换为SQL查询。谢谢@Dennis!仅仅通过linq到entitites就可以做到这一点吗?我的意思是,“from”、“group”和“select”子句是什么?这是LINQtoEntities,只是另一种表单方法语法(而您的示例使用查询语法)。谢谢@Dennis!仅仅通过linq到entitites就可以做到这一点吗?我的意思是,有“from”、“group”和“select”子句吗?这是LINQtoEntities,只是另一种表单方法语法(而您的示例使用查询语法)。谢谢。这是可行的,但应用程序必须是列表,而不是列表。它只需要从g
!:)的项中提取Application
属性最好不要将Linq-to-Object(扩展)方法调用(如ToList
)与Linq-to-Entity查询混淆。而是迭代Linq到实体查询,并将模型转换为(比如)视图模型对象(如果您进行更多的编码并以惰性的方式从数据库中提取它们,而不是将它们全部拉入内存,这将是完美的;您将看到分页等任务的不同,在这种情况下,不需要完整的答案)。谢谢。这是可行的,但应用程序必须是列表,而不是列表。它只需要从g
!:)的项中提取Application
属性,最好不要将诸如ToList
之类的Linq to Object(扩展)方法调用与Linq to Entity查询混淆。而是迭代Linq到实体查询,并将模型转换为(比如)视图模型对象(如果您进行更多的编码并以一种懒惰的方式从数据库中提取它们,而不是将它们全部拉入内存,那将是完美的;您将看到分页之类的任务的区别,在这种情况下,不需要完整的答案集)。