linq to EF分組查詢 group by 的使用

第一種：查詢表達式語法：

IQueryable<EnrollmentDateGroup> data = from student in db.Students group student by student.EnrollmentDate into dateGroup select new EnrollmentDateGroup() { EnrollmentDate = dateGroup.Key, StudentCount = dateGroup.Count() }; return View(data.ToList());
第二種：方法labmda語法：

var context = new SiteDataContext();
    var notifications = context.Notifications
        .Where(n => n.UserId == userId)
        .Where(n => !n.IsDismissed)
        .GroupBy(n => n.NotificationType)
        .Select(g => new NotificationViewModel { Count = g.Count(), NotificationType = g.Key.ToString(), BadgeClass = NotificationType.Email == g.Key ? "success" : "info" });


實際使用過的例子：使用的是方法+labmda 

[ChildActionOnly]
public ActionResult IndexCourseOnClickRange()
{
var courseList =new List<IndexClickRangeOnCourseViewModel>();  //先建立一個list .
var _coursesByClick = _resourceService.FindAll().Where(r => r.ResourceIsShow == true) //也可以使用ContextFactory.GetCurrentContext().Source取得課程資源。
.GroupBy(r => r.CourseID)
.Select(g => new IndexClickRangeOnCourseViewModel
{
CourseID = g.Key,
CourseResourceCount =g.Count(),
CourseResourceTotalClickCount = g.Sum(x =>x.ClickCount) //對分組后的數據 按點點擊量匯總。
});

var _courses = _coursesByClick.ToList(); //這里是重點，必須使用tolist方法來立即執行，從數據庫中獲取值，如果不立即執行，延遲執行將會發生錯誤。

foreach (var item in _courses)
{
Course course =_courseService.Find(item.CourseID);;
courseList.Add(new IndexClickRangeOnCourseViewModel { CourseID = item.CourseID, CourseName = course.CourseName, CourseType=course.CourseType, CourseResourceCount=item.CourseResourceCount , CourseDescription =course.CourseDescription, PriorOrder = course.PriorOrder, CourseResourceTotalClickCount = item.CourseResourceTotalClickCount });
}
// var courses = _coursesByClick.ToList() as IEnumerable<Course>;
// var courses = _coursesByClick.OrderByDescending(c => c.CourseResourceTotalClickCount) as IQueryable<IndexClickRangeOnCourseViewModel>;
var coursesOrder =courseList.OrderByDescending(c => c.CourseResourceTotalClickCount).ThenBy(c => c.PriorOrder) as IEnumerable<IndexClickRangeOnCourseViewModel>; //排序后變成了IOrder ,為了使用take取前面的序列，必須轉換成IEnumerable ;
if(coursesOrder.Count() >=7)
{
coursesOrder =coursesOrder.Take(7);

}

return PartialView("_IndexCourseOnClickRange", coursesOrder);
}