How do I aggregate joins?

I have three related tables:

Employee(EmployeeId, EmployeeName)

Skill(SkillId, SkillName)

EmployeeSkill(EmployeSkillId, EmployeeId, SkillId)

EmployeSkillId is an identity.

The rows in the database are the following:

Employee Table:

EmployeeId | EmployeeNumber | EmployeeName
---------- | -------------- | ------------
        1  |         10015  |  John Doe

Skill Table:

SkillId | SkillName
------- | ---------
     1  |  .NET
     2  |  SQL
     3  |  OOD
     4  |  Leadership

EmployeeSkill Table:

EmployeeSkillId | EmployeeId | SkillId
--------------- | ---------- | -------
             1  |         1  |      1
             2  |         1  |      2
             3  |         1  |      3
             4  |         1  |      4

If I have an employee who has three skills registered on EmployeSkill, I'd like to be able t开发者_JAVA百科o have a result as the following:

John Doe, "Skill-1, Skill2, Skill-3"

That is, to concatenate the name of the skills for that employee into a single string.

I tried the following, but it isn't working.

var query = from emp in Employee.All()
            from es in emp.EmployeeSkills
            join sk in Skill.All() on es.SkillId equals sk.SkillId
            group sk by new {emp.EmployeeName} into g
            select new TestEntity
            {
                Name = g.Key.EmployeeName,
                Skills = g.Aggregate(new StringBuilder(),
                                     (sb, grp_row) => sb.Append(grp_row.SkillName))
                          .ToString()
            };

The aggregated list of skill names is coming back empty. How can I do this?

---

It sounds like you could do the join as part of the select:

var query = from emp in Employee.All()
            select new TestEntity {
                Name = emp.EmployeeName,
                Skills = string.Join(", ", 
                       (from es in emp.EmployeeSkills
                        join sk in Skill.All() on es.SkillId equals sk.SkillId
                        select sk.SkillName)) };

Now that's going to do the join separately for each individually, which isn't terribly efficient. Another option is to build a mapping from skill ID to skill name first:

var skillMap = Skill.All().ToDictionary(sk => sk.SkillId,
                                        sk => sk.SkillName);

then the main query is easy:

var query = from emp in Employee.All()
            select new TestEntity {
                Name = emp.EmployeeName,
                Skills = string.Join(", ",
                       emp.EmployeeSkills.Select(sk => skillMap[sk.SkillId]))};

Ultimately there are lots of ways of skinning this cat - for example, if you wanted to stick to your original approach, that's still feasible. I would do it like this:

var query = from emp in Employee.All()
            from es in emp.EmployeeSkills
            join sk in Skill.All() on es.SkillId equals sk.SkillId
            group sk.SkillName by emp into g
            select new TestEntity
            {
                Name = g.Key.EmployeeName,
                Skills = string.Join(", ", g)
            };

At this point it's quite similar to your original query, just using string.Join instead of Aggregate, of course. If all these three approaches come back with an empty skills list, then I suspect there's something wrong with your data. It's not obvious to me why your first query would "succeed" but with an empty skill list.

EDIT: Okay, here's a short(-ish) but complete example of it working:

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

public class Employee
{
    public int EmployeeId { get; set; }
    public string EmployeeName { get; set; }

    public static List<Employee> All { get; set; }

    public IEnumerable<EmployeeSkill> EmployeeSkills
    {
        get 
        { 
            return EmployeeSkill.All
                           .Where(x => x.EmployeeId == EmployeeId);
        }
    }
}

public class Skill
{
    public string SkillName { get; set; }
    public int SkillId { get; set; }

    public static List<Skill> All { get; set; }
}

public class EmployeeSkill
{
    public int SkillId { get; set; }
    public int EmployeeId { get; set; }

    public static List<EmployeeSkill> All { get; set; }
}

class Test
{
    static void Main()
    {
        Skill.All = new List<Skill>
        {
            new Skill { SkillName = "C#", SkillId = 1},
            new Skill { SkillName = "Java", SkillId = 2},
            new Skill { SkillName = "C++", SkillId = 3},
        };

        Employee.All = new List<Employee>
        {
            new Employee { EmployeeName = "Fred", EmployeeId = 1 },
            new Employee { EmployeeName = "Ginger", EmployeeId = 2 },
        };

        EmployeeSkill.All = new List<EmployeeSkill>
        {
            new EmployeeSkill { SkillId = 1, EmployeeId = 1 },
            new EmployeeSkill { SkillId = 2, EmployeeId = 1 },
            new EmployeeSkill { SkillId = 2, EmployeeId = 2 },
            new EmployeeSkill { SkillId = 3, EmployeeId = 2 },
        };

        var query = from emp in Employee.All
            from es in emp.EmployeeSkills
            join sk in Skill.All on es.SkillId equals sk.SkillId
            group sk.SkillName by emp.EmployeeName into g
            select new
            {
                Name = g.Key,
                Skills = string.Join(", ", g)
            };

        foreach (var result in query)
        {
            Console.WriteLine(result);
        }
    }
}

Results:

{ Name = Fred, Skills = C#, Java }
{ Name = Ginger, Skills = Java, C++ }