Question

假设我有以下域名（不是我的实际域名，而是一个简单的例子）

public class ClassRoom
{
  public int Id {get;set;}
  public string Name {get;set;}
  public virtual ICollection<Desk> Desks{get;set;}
  public virtual ICollection<LunchBox> LunchBoxs{get;set;}
}

public class Desk
{
  public int Id {get;set;}
  public String Colour {get;set;}
  public String Make {get;set;}
  Public ClassRoom ClassRoom {get;set;}
}

public class LunchBox
{
  public int Id {get;set;}
  public String Colour {get;set;}
  public int Volume {get;set;}
}

我希望能够查询如下

“给我所有有蓝色办公桌或绿色午餐盒的课程”，返回班级列表

我原本认为代码是

var efClasses = (from d in myentity.ClassRooms
                  where ( (d.Desks.Colour == "blue")
                         || (d.LunchBoxs.Colour == "green) )
select d).Distinct();

这样做会给我带来错误

无法将lambda表达式转换为类型'string'，因为它不是委托类型（显然对Color不满意，它希望这里有一个方法

所以我把它改成了这个实验

var efClasses = (from d in myentity.ClassRoom
                  where ( (d.Desks.Where(x=>x.Colour == "blue"))
                         || (d.LunchBoxs.Where(x=>x.Colour == "green)) )
select d).Distinct();

但这也不会编译（事后我有一种感觉，我在试图运气。

我可以反过来改变我的myentity.Desks和myEntity.Lunchboxs，并返回两个单独的ClassRooms列表，然后找到联合，但这将需要2个单独的DB命中，并且感觉哈克

Answer 1

而不是使用count＆gt; 0，你可以使用Any

var efClasses = (from d in myentity.ClassRoom
                 where (d.Desks.Any(x => x.Colour == "blue") ||
                        d.LunchBoxs.Any(x => x.Colour == "green"))
                 select d);

Answer 2

像这样使用Count＆gt; 0。

 var efClasses = (from d in myentity.ClassRoom
                             where (d.Desks.Count(x => x.Colour == "blue") > 0
                                    || d.LunchBoxs.Count(x => x.Colour == "green") > 0)
                             select d);

实体框架基于跨多个子集合的过滤器选择对象

2 个答案: