c# - Reduce 不能在分组中包含 Average() 方法

刚刚升级到 v2，这不再有效；如果我尝试使用 Count()，我会收到类似的错误

public class Deck_Ratings : AbstractIndexCreationTask<DeckRating, Deck_Ratings.ReduceResult>
{
    public class ReduceResult
    {
        public string DeckId { get; set; }
        public int Rating { get; set; }
    }

    public Deck_Ratings()
    {
        Map = deckRatings => deckRatings.Select(deckRating => new
                             {
                                 deckRating.DeckId,
                                 deckRating.Rating
                             });
        Reduce = reduceResults => reduceResults
                                      .GroupBy(reduceResult => reduceResult.DeckId)
                                      .Select(grouping => new
                                              {
                                                  DeckId = grouping.Key,
                                                  Rating = grouping.Average(reduceResult => reduceResult.Rating)
                                               });
    }
}

最佳答案

禁止可能受缩减批次大小影响的聚合(例如 Count 和 Average)，因为它们会产生错误的结果。您可能已经能够在 1.0 以下使用它，但是您的平均值可能是错误的，除非您的项目太少以至于它们都在一个缩减批次中完成。要了解有关减少批次的更多信息，请阅读 Map / Reduce - A Visual Explanation

您必须通过为每个项目求和 1 来计算项目。您必须通过将值的总和作为总数，将 1 的总和作为计数，然后将它们相除来平均项目。

public class Deck_Ratings : AbstractIndexCreationTask<DeckRating, Deck_Ratings.ReduceResult>
{
    public class ReduceResult
    {
        public string DeckId { get; set; }
        public int TotalRating { get; set; }
        public int CountRating { get; set; }
        public double AverageRating { get; set; }
    }

    public Deck_Ratings()
    {
        Map = deckRatings => deckRatings.Select(deckRating => new
                             {
                                 deckRating.DeckId,
                                 TotalRating = deckRating.Rating,
                                 CountRating = 1,
                                 AverageRating = 0
                             });
        Reduce = reduceResults => reduceResults
                                      .GroupBy(reduceResult => reduceResult.DeckId)
                                      .Select(grouping => new
                                              {
                                                  DeckId = grouping.Key,
                                                  TotalRating = grouping.Sum(reduceResult => reduceResult.TotalRating)
                                                  CountRating = grouping.Sum(reduceResult => reduceResult.CountRating)
                                               })
                                      .Select(x => new
                                               {
                                                  x.DeckId,
                                                  x.TotalRating,
                                                  x.CountRating,
                                                  AverageRating = x.TotalRating / x.CountRating
                                               });
    }
}

关于c# - Reduce 不能在分组中包含 Average() 方法，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/14233875/