Linq平均查詢和投影

Question

我正在嘗試從評分表中獲得人們的平均評分，我對此有以下代碼。 我將結果投影到PersonVm類中

    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;                   
    }

我收到以下錯誤

無法將lambda表達式轉換為委托類型'System.Func'，因為該塊中的某些返回類型不能隱式轉換為委托返回類型

和

無法隱式轉換類型“ double”？ 到“布爾” ArtsHub.DataLayer

請問我的平均代碼有什么問題？

Answer 1

此x=>x.Ratings.Any(y=>y.RatingValue)返回bool作為結果，因為.Any()產生boolean結果類型，因此您需要為.Average()獲得一個lambda ，后者產生的type相同作為Rating屬性的類型。

看來Rating是double? 所以您需要一個lambda傳遞給.Average() ，該返回一個double? 或double或某些東西可以轉換為double?

我認為你需要這個：

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

Answer 2

您需要將它們添加到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; 

Answer 3

注意... .GroupBy(item => new ...之前的.ToList()導致您的數據首先在內存中獲取，而group by將在內存中完成，您可以比較這兩個查詢的執行時間。

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();