在运行时按日/周/月/年动态分组

编码器101

我想根据传递给函数的int对查询进行不同的分组。目前，我有这个非常棘手的解决方案：

`.GroupBy(occurrence => new { date = 
                // Bucket by day.
                timeBucket == 0 ? DbFunctions.TruncateTime(occurrence.occurrenceDate) : 
                // Bucket by week.
                timeBucket == 1 ? DbFunctions.AddDays(DbFunctions.CreateDateTime(occurrence.occurrenceDate.Year, 1, 1, 0, 0, 0), 7*(occurrence.occurrenceDate.DayOfYear/7)) : 
                // Bucket by month.
                timeBucket == 2 ? DbFunctions.TruncateTime(DbFunctions.CreateDateTime(occurrence.occurrenceDate.Year, occurrence.occurrenceDate.Month, 1, 1, 1, 1)) :
                // Bucket by year.
                DbFunctions.TruncateTime(DbFunctions.CreateDateTime(occurrence.occurrenceDate.Year, 1, 1, 1, 1, 1)),
                type = occurrence.type })`

我如何计算日期的细节对我来说并不重要（但随时可以提供帮助）。我想避免不必对数据库中的每一行都执行此case语句。无论如何要避免这样做？我已经尝试了多种解决方案，例如使用表达式，但是我无法返回想要的对象，因为表达式树必须是无参数的...

如果有人有解决方案，将不胜感激。

谢谢！

格特·阿诺德

定义一个石斑鱼类：

class TimeGrouper
{
    public int Year { get; set; }
    public int Month { get; set; }
    public int Week { get; set; }
    public int Day { get; set; }
}

还有一个返回表达式的函数：

using System.Data.Entity.SqlServer;
...
Expression<Func<Occurrence, TimeGrouper>> GetGrouper(int grouping)
{
    switch (grouping)
    {
        case 1:
            return o => new TimeGrouper
                        { 
                            Year = o.occurrenceDate.Year
                        };
        case 2:
            return o => new TimeGrouper 
                        { 
                            Year = o.occurrenceDate.Year,
                            Month = o.occurrenceDate.Month
                        };
        case 3:
            return o => new TimeGrouper 
                        {
                            Year = o.occurrenceDate.Year,
                            Week = SqlFunctions.DatePart("wk", o.StartDate).Value
                        };
        default:
            return o => new TimeGrouper
                        {
                            Year = o.occurrenceDate.Year,
                            Day = SqlFunctions.DatePart("dy", o.StartDate).Value
                        };
    }
}

现在你可以打电话