根据日,周和月过滤数据

Question

我想过滤今天,周和月的数据.

var data = new DashboardViewModel()
           {
               Today = new State() { Download = db.Download.Where(x => x.Time == DateTime.Today).Count(), Visit = db.Visit.Where(x => x.Time == DateTime.Today).Count() },
               Week = new State() { Download = db.Download.Where(x => x.Time.DayOfYear <= Day && x.Time.DayOfYear >= Day - 7).Count(), Visit = db.Visit.Where(x => x.Time.DayOfYear <= Day && x.Time.DayOfYear >= Day - 7).Count() },
               Month = new State() { Download = db.Download.Where(x => x.Time.DayOfYear <= Day && x.Time.DayOfYear >= Day - 30).Count(), Visit = db.Visit.Where(x => x.Time.DayOfYear <= Day && x.Time.DayOfYear >= Day - 30).Count() },
               Total = new State() { Download=db.Download.Count(),Visit=db.Visit.Count()}
           };

它给出了错误:

LINQ to Entities不支持指定的类型成员'DayOfYear'.

如何解决此错误,还是有其他更好的方法来解决此问题？

Answer 1

问题是，当 LINQ to Entities 解析 lambda 表达式并将其转换为 SQL 时，它不知道“DayOfYear”是什么，以及它如何映射到有效的 SQL。这就是DbFunctions（ v6 之前的 EF 中的EntityFunctions）发挥作用的地方，它允许您在 LINQ to Entities 中使用日期操作函数。例如，要按周过滤，您可以使用：

Week = new State() { 
    Download = db.Download.Where(x => DbFunctions.DiffDays(DbFunctions.TruncateTime(x.Time), Day) <= 7).Count(), 
    Visit = db.Visit.Where(x => DbFunctions.DiffDays(DbFunctions.TruncateTime(x.Time), Day) <= 7).Count() }

TruncateTime 删除日期中的时间部分，DiffDays(d1, d2) 给出 2 天之间的差值 (d2 - d1)。