LINQ到SQL查询

Question

我有一个LINQ查询，它检索所有用户及其角色：

var userRoles = from u in db.GetTable<User>()
                            join ur in db.GetTable<UserRole>()
                                on u.UserID equals ur.UserID
                            join r in db.GetTable<Role>()
                                on ur.RoleID equals r.RoleID
                                orderby u.UserID
                            select new
                                       {
                                           u.UserID,
                                           r.RoleName
                                       };

系统中的用户可以具有多个角色。此查询的结果(以表格格式表示)如下所示：

1行政

1名雇员

2名雇员

3名雇员

如何重写此查询以以逗号分隔的值返回所有用户角色，如：

1行政，雇员

2名雇员

3名雇员

Answer 1

嗨，库马尔，我创建了一个小型控制台应用程序来模仿我相信你拥有的数据。我认为它显示了你想要的行为。这不是世界上最伟大的代码，但我认为算法才是重点。我刚刚对ToString()进行了快速覆盖，以正确显示数据。

我所做的主要更改是为要显示的数据创建一个已定义的类，并将linq查询分成两个单独的部分：

using System;
using System.Collections.Generic;
using System.Linq;

namespace Test
{
    class Program
    {
        static void Main()
        {
            var users = new List<User>
                            {
                                new User
                                    {
                                        UserID = "1"
                                    },
                                new User
                                    {
                                        UserID = "2"
                                    },
                                new User
                                    {
                                        UserID = "3"
                                    }
                            };

            var roles = new List<Role>
                            {
                                new Role
                                    {
                                        RoleID = "1",
                                        RoleName = "Admin"
                                    },
                                new Role
                                    {
                                        RoleID = "2",
                                        RoleName = "Employee"
                                    }
                            };

            var userRoles = new List<UserRole>
                                {
                                    new UserRole
                                        {
                                            UserID = "1",
                                            RoleID = "1"
                                        },

                                    new UserRole
                                        {
                                            UserID = "1",
                                            RoleID = "2"
                                        },

                                    new UserRole
                                        {
                                            UserID = "2",
                                            RoleID = "2"
                                        },

                                    new UserRole
                                        {
                                            UserID = "3",
                                            RoleID = "2"
                                        }
                                };

            var userRoles2 = from u in users
                             orderby u.UserID
                             select new UserList
                             {
                                 UserID = u.UserID,
                                 Roles = (from r in roles
                                            join ur in userRoles
                                            on u.UserID equals ur.UserID
                                            where ur.RoleID == r.RoleID
                                            select r).ToList()
                             };

            foreach (var item in userRoles2)
            {
                Console.WriteLine(item);
            }
            Console.ReadKey();
        }
    }

    public class User
    {
        public string UserID;
    }

    public class UserRole
    {
        public string UserID;
        public string RoleID;
    }

    public class Role
    {
        public string RoleID;
        public string RoleName;
    }

    public class UserList
    {
        public string UserID;
        public List<Role> Roles;

        public override string ToString()
        {
            string output = UserID + " ";
            foreach (var role in Roles)
            {
                output += role.RoleName + ", ";
            }
            output = output.Substring(0, output.Length - 2);
            return output;
        }
    }
}

Answer 2

这是一种方法，还没有测试过：

    from u in db.GetTable<User>()   
                      join ur in db.GetTable<UserRole>()   
                            on u.UserID equals ur.UserID   
                      join r in db.GetTable<Role>()   
                            on ur.RoleID equals r.RoleID   
                      orderby u.UserID
                      group u by u.UserID into g    
                      select new   
                                {   
                                    UserId = g.Key,   
                                    Roles = String.Join (" ,", g.UserRoles.SelectMany(c => c.Roles).Select(p=> p.RoleName).ToArray()))
                                };