Linq 平均查询和投影

Question

我正在尝试从评分表中获取人们的平均评分，我有以下代码。我将结果投影到 PersonVm 类中

    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

无法将 lambda 表达式转换为委托类型“System.Func”，因为块中的某些返回类型不能隐式转换为委托返回类型

和

不能隐式转换类型'double'？到“布尔”ArtsHub.DataLayer

请问我的平均代码有什么问题？

Answer 1

注意.ToList()之前... .GroupBy(item => new ...是导致你的数据先在内存中获取，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

这个x=>x.Ratings.Any(y=>y.RatingValue)返回一个bool作为结果，因为.Any()产生boolean结果类型，你需要得到一个lambda到.Average()，它产生一个type与@的类型相同987654328@财产。

似乎Rating 是double?，所以你需要将lambda 传递给.Average()，它返回double? 或double 或者可以转换为double? 的东西

我认为你需要这个：

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

Answer 3

您需要将它们添加到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;