Linq to Sql，使用一个查询检索嵌套的数据结构 [英] Linq to Sql, retrieving nested data structures with one query

问题描述

我不确定是否已经找到答案，因为我无法确定Linq中的哪种机制适用于这种情况，或者我是否需要手动完成.

I'm not sure if an answer for this already exists, as I can't figure out what mechanism in Linq is meant for this situation, or if I just need to do it manually.

假设我有2张桌子:

------------Employees-------------
EmployeeID  Name  -other columns-

   ---------EmployeeSkills-----------
   EmployeeID  Skill  -other columns-

因此，每个员工可以拥有0个或更多技能.

So each employee can have 0 or more skills.

我的目标是使用一个sql查询将这些信息绘制到内存中的数据结构中

My goal is to draw this information into a data structure in memory, using one sql query

    class StaticEmployee
    {
        int EmployeeID;       
        string Name;
        List<string> Skills;
    }
    List<StaticEmployee> employees = (???).ToList();

由于已经建立了表关系，所以我只能为每个雇员表(从db中的雇员中选择雇员)，但是当我访问EmployeeSkills属性时，它将执行一个单独的查询每条员工记录，因为第一个查询未返回该数据.

Since my table relations are setup, I could just foreach the employees table (from employee in db.Employees select employee), however when I access the EmployeeSkills property, it is going to execute a seperate query for every employee record since that data wasn't returned with the first query.

或者，就Sql而言，我想做的是运行带有左联接的查询:

Alternatively, and I what I want done so far as Sql is concerned, is run a query with a left join:

SELECT Employees.EmployeeID, Employees.Name, EmployeeSkills.Skill FROM Employees LEFT JOIN EmployeeSkills ON Employees.EmployeeID=EmployeeSkills.EmployeeID

那将获得我的列表，但是我需要手动将其整理到我的列表中，因为具有2个技能的员工将返回2行. 是否有执行此操作的Linq操作? 下面的假设示例

That will get me my list, but I'll need to manually collate it into my list since an employee with 2 skills will return 2 rows. Is there a Linq operation that does this? Hypothetical example below

from employee in db.Employees
select new 
{
   EmployeeID = employee.EmployeeID, 
   Name = employee.Name,
   Skills = new List(
                     from employeeSkill in employee.EmployeeSkills 
                     select employeeSkill.skill
                    )
}

推荐答案

var options = new DataLoadOptions();
options.LoadWith< Employees>(e => e.EmployeeSkills);
context.LoadOptions = options;
//retrieve employees, they will come loaded with EmployeeSkills

如果要改用linq查询版本，这也会加载一次:

If you were to use the linq query version instead, this will load them once as well:

from employee in db.Employees
select new 
{
   EmployeeID = employee.EmployeeID, 
   Name = employee.Name,
   Skills = employee.EmployeeSkills
}

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