c# - 如何在 Linq 查询中获取分组结果的平均值

Question

我有一个获取过去 12 个月评分的 linq 查询:

var ratingsByMonth =
    from month in Enumerable.Range(0, 12)
    let key = new { Year = DateTime.Now.AddMonths(-month).Year, Month = DateTime.Now.AddMonths(-month).Month }
    join groupedRating in ratings on key
            equals new
            {
                groupedRating.RatingDate.Year,
                groupedRating.RatingDate.Month
            } into g
    select new {
        Date = key,
        Rating = g.Select(x=>x.Rating)
    };

ratingsByMonth 从我的数据集中返回以下 JSON:

{Date: {Year: 2016, Month: 11}, Rating: [4]}
{Date: {Year: 2016, Month: 10}, Rating: [5, 5, 4]}
{Date: {Year: 2016, Month: 9}, Rating: []}
{Date: {Year: 2016, Month: 8}, Rating: []}
{Date: {Year: 2016, Month: 7}, Rating: []}
{Date: {Year: 2016, Month: 6}, Rating: []}
{Date: {Year: 2016, Month: 5}, Rating: []}
{Date: {Year: 2016, Month: 4}, Rating: []}
{Date: {Year: 2016, Month: 3}, Rating: []}
{Date: {Year: 2016, Month: 2}, Rating: []}
{Date: {Year: 2016, Month: 1}, Rating: []}
{Date: {Year: 2015, Month: 12}, Rating: []}

不过，我正在尝试将评级字段汇总为平均值。我试过这个:

return Json(new
{
    Average = ratingsByMonth.Average(x=>x.Rating),
    Dates = ratingsByMonth.Select(x => Date)
});

但是我在平均值 - 行上突出显示了以下错误:

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable' to 'long?'

我正在寻找的输出是:

{Date: {Year: 2016, Month: 11}, Rating: 4}
{Date: {Year: 2016, Month: 10}, Rating: 3.5}
{Date: {Year: 2016, Month: 9}, Rating: 0}
{Date: {Year: 2016, Month: 8}, Rating: 0}
{Date: {Year: 2016, Month: 7}, Rating: 0}
{Date: {Year: 2016, Month: 6}, Rating: 0}
{Date: {Year: 2016, Month: 5}, Rating: 0}
{Date: {Year: 2016, Month: 4}, Rating: 0}
{Date: {Year: 2016, Month: 3}, Rating: 0}
{Date: {Year: 2016, Month: 2}, Rating: 0}
{Date: {Year: 2016, Month: 1}, Rating: 0}
{Date: {Year: 2015, Month: 12}, Rating: 0}

编辑:

已尝试更改为以下内容:

return Json(new
{
    Average = ratingsByMonth.Select(x => x.Rating).SelectMany(x => x).Average(),
    Dates = ratingsByMonth.Select(x => x.Date.Month)
});

但是，这只为我提供了整个数据集中的单个“平均值”值，而不是每个日期的平均值。

编辑:

这是示例数据:

RatingID    Rating  RatingDate
5           5   2016-10-09 20:11:29.590
11          0   2016-10-09 20:14:42.503
21          5   2016-10-09 20:24:15.667
60          4   2016-10-28 11:01:21.220
64          4   2016-11-03 16:30:47.657

所以 11 月的平均值应该是 4，10 月的平均值应该是 3.5 (5+0+5+4/4)

Answer 1

在原始查询中，您可能想要这样做。

//other code removed for brevity
select new {
    Date = key,
    Rating = g.Count() > 0 ? g.Average(x => x.Rating) : 0
};

然后返回结果

return Json(ratingsByMonth);