LINQ到實體：多對多在一個單一的選擇

Question

我有這個域名：

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 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; } 
} 

我希望做一個選擇返回類似的東西：

List of select new 
{ 
    User = user, 
    Applications = applications // List of all user's applications 
} 

我嘗試：

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 
} 

但是，這會重複用戶到每個應用程序。

我知道我可以在兩個select語句中做到這一點，但我不希望這樣做。

我該怎麼做？

Answer 1

我不記得，如果實體框架可以基於實體對象本身做groupby（並且在場景後面提取它Id並且替換適合的東西等等）;但是這個代碼適用於這種情況：

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) }; 

如果你願意有User已經提取：

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) }; 

假設你對一個實體框架上下文編寫查詢 - 而不是隻是想做一個Linq to Objects查詢。

Answer 2

試試這個：

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() 
     }); 

Answer 3

其實，你只需要通過用戶設置組UserApplications實體：

context 
    .UserApplications 
    .GroupBy(_ => _.User, _ => _.Application) 
    .ToList(); 

，因爲，事實上，IGrouping<User, Application>是你所需要的（Key是用戶，組項目是他的應用程序）。

任何其他的改進是品味的問題，像投影到匿名類型：

context 
    .UserApplications 
    .GroupBy(_ => _.User, _ => _.Application) 
    .Select(_ => new 
    { 
     User = _.Key, 
     // since IGrouping<User, Application> is IEnumerable<Application>, 
     // we colud return a grouping directly 
     Applications = _ 
    }) 
    .ToList(); 

（另一種投影選項扔掉在Applications組密鑰）：

context 
    .UserApplications 
    .GroupBy(_ => _.User, _ => _.Application) 
    .Select(_ => new 
    { 
     User = _.Key, 
     Applications = _.Select(app => app) 
    }) 
    .ToList();