LINQ 查询连接多个表并在单行中获取逗号分隔值

Question

我有下表的值。

帐户：

ID	姓名	Email
101	纳西尔·乌丁	nasir@email.com

角色：

ID	标题
101	行政
102	操作员

账户角色：

帐户ID	角色 ID
101	101
101	102

现在我想写一个 linq 得到如下结果：

用户帐号

帐户ID	姓名	Email	角色
101	纳西尔·乌丁	nasir@email.com	管理员、操作员

为了得到上述结果，我在 LINQ 中编写了以下查询。 但它没有得到预期的结果。

var userAccount1 = (from account in _db.Accounts
                               join accountRole in _db.AccountRoles on account.Id equals accountRole.AccountId
                               join role in _db.Roles on accountRole.RoleId equals role.Id             
                               select new UserAccountInfo
                               {
                                   AccountId = account.Id,
                                   Name = account.UserFullName,
                                   Email = account.Email,                                 
                                   Roles = string.Join(",", role.Title)
                               });

Answer 1

我终于找到了我的答案。 结果可以通过不同的方式实现。 示例如下：

var answer1 = (from account in userAccounts
    join accountRole in accountRoles on account.Id equals accountRole.AccountId
    join role in roles on accountRole.RoleId equals role.Id
    select new UserAccount
    {
        AccountId = account.Id,
        Name = account.Name,
        Email = account.Email,
        Roles = role.Title
    }).ToList().GroupBy(x => new { x.AccountId, x.Name, x.Email }).Select(y => new UserAccount
    {
        AccountId = y.Key.AccountId,
        Name = y.Key.Name,
        Email = y.Key.Email,
        Roles = string.Join(", ", y.Select(a => a.Roles))
    }).ToList();
----------------------------------------------------------------------------
var answer2 = (from account in userAccounts
     join accountRole in accountRoles on account.Id equals accountRole.AccountId
     join role in roles on accountRole.RoleId equals role.Id
     group new { account, role } by new { account.Id, account.Name, account.Email } into ag
     select new UserAccount
     {
         AccountId = ag.Key.Id,
         Name = ag.Key.Name,
         Email = ag.Key.Email,
         Roles = string.Join(", ", ag.Select(x=> x.role.Title))
     }).ToList();
    ----------------------------------------------------------------------------

var answer3 = (from account in userAccounts
     let roles1 = from accountRole in accountRoles
                  join role in roles on accountRole.RoleId equals role.Id
                  where accountRole.AccountId == account.Id
                  select role
     select new UserAccount
     {
         AccountId = account.Id,
         Name = account.Name,
         Email = account.Email,
         Roles = string.Join(", ", roles1.Select(x => x.Title))
     }).ToList();

Answer 2

下面根据您在帖子中提供的信息使用 Lambda 表达式（不是查询表达式）为您提供预期结果（以及一些假设，因为我在您的任何表中都找不到例如UserFullName ）

注意：我也相信有一种更有效的方法可以做到这一点，但如果没有别的，这只是一个起点。

（这里正在工作 .NET 下面的小提琴： https://dotnetfiddle.net/aGra15 ）：

    // Join the AccountRoles and Roles together and group all Titles for
    // a given AccountId together
    var groupedAccountRoles = AccountRoles.GroupJoin(Roles, i => i.RoleId, o => o.Id, (o, i) => new {o, i})
        .Select(x => new {AccountId = x.o.AccountId, Titles = string.Join(",", x.i.Select(y => y.Title))});

    // Perform another GroupJoin to group by AccountId and Join to groupedAccountRoles table. Then `string.Join()`
    var userAccount1 = Accounts.GroupJoin(AccountRoles, acc => acc.Id, accrol => accrol.AccountId,
            (o, i) => new {o, UserAccountRoles = i})
        .GroupJoin(groupedAccountRoles, ii => ii.o.Id, oo => oo.AccountId,
            (ii, oo) => new UserAccountInfo
            {
                AccountId = ii.o.Id, 
                Email = ii.o.Email, 
                Name = ii.o.Name,
                Roles = string.Join(",", oo.Select(x => x.Titles))
            });

这将给出以下 output：