Linq的平均查询和投影

Question

我试图从评级表中获得人的平均评分，我有以下代码。我伸出结果为PersonVm类

public IEnumerable SrchArtisansByBSSLR(string q, int s = 0, int l = 0, double z = 0) 
    { 
     var list = from item in _entities.People 
        where item.Skill.SkillName == q && item.StateId == s && item.LGAId == l && item.Ratings.Any(g => g.RatingValue == z) 
        group item by item.PersonId into d 
        select new PersonVm 
        { 
         PersonId = d.Key, 
         Rating = d.Average(x=>x.Ratings.Any(y=>y.RatingValue)), 
         LastName = item.LastName, 
         Skill = item.Skill.SkillName, 
         StateId = item.StateId, 
         LGAId = item.LGAId, 
         Address = item.Address, 
         Email = item.Email, 
         ImageSource = @"\Photos\" + item.ImageSource, 
         RegDate = item.RegDate      
        }; 
     return list;     
    } 

我收到以下错误

无法转换lambda表达式的委托类型“System.Func”，因为在一些块的返回类型不隐式转换为委托返回类型

和

不能隐式转换类型'double？' to'bool'ArtsHub.DataLayer

请问我的代码获得平均值有什么问题？

Answer 1

注.ToList()... .GroupBy(item => new ...之前引起你的数据在内存中先取出，并group by会在内存中完成，你可以比较这两个查询的执行时间。

var list = _entities.People 
      .Where(item => item.Skill.SkillName == q && 
          item.StateId == s && 
          item.LGAId == l && 
          item.Ratings.Any(g => g.RatingValue == z)) 
      .ToList() 
      .GroupBy(item => new 
      { 
       item.PersonId, 
       item.LastName, 
       item.Skill.SkillName, 
       item.StateId, 
       item.LGAId, 
       item.Address, 
       item.Email, 
       item.ImageSource, 
       item.RegDate 
      }) 
      .Select(d => new PersonVm() 
      { 
       PersonId = d.Key, 
       Rating = d.Average(x => x.Ratings.Any(y => y.RatingValue)), 
       LastName = d.Key.LastName, 
       Skill = d.Key.SkillName, 
       StateId = d.Key.StateId, 
       LGAId = d.Key.LGAId, 
       Address = d.Key.Address, 
       Email = d.Key.Email, 
       ImageSource = @"\Photos\" + d.Key.ImageSource, 
       RegDate = d.Key.RegDate 
      }) 
      .ToList(); 

Answer 2

这x=>x.Ratings.Any(y=>y.RatingValue)返回bool的结果，因为.Any()产生boolean结果类型，你需要获得一个lambda到.Average()，其产生type相同类型Rating属性。

似乎Rating是一个double?所以你需要一个lambda被传递到.Average()它会返回一个double?或double或东西可以转化为double?

我认为你需要这样的：

Rating = d.Average(x => x.Ratings.Sum(y => y.RatingValue)) 

Answer 3

你需要将它们添加到group by。

var list = from item in _entities.People 

      where item.Skill.SkillName == q && item.StateId == s && item.LGAId == l && item.Ratings.Any(g => g.RatingValue == z) 

      group item by new { 
       item.PersonId, 
       item.LastName, 
       item.Skill.SkillName, 
       item.StateId, 
       item.LGAId, 
       item.Address, 
       item.Email, 
       item.ImageSource, 
       item.RegDate 
      } into d 

      select new PersonVm 
      { 
       PersonId = d.Key, 
       Rating = d.Average(x => x.Ratings.Any(y => y.RatingValue)), 
       LastName = d.Key.LastName, 
       Skill = d.Key.SkillName, 
       StateId = d.Key.StateId, 
       LGAId = d.Key.LGAId, 
       Address = d.Key.Address, 
       Email = d.Key.Email, 
       ImageSource = @"\Photos\" + d.Key.ImageSource, 
       RegDate = d.Key.RegDate 
      }; 
return list;