linq to EF分组查询 group by 的使用

第一种：查询表达式语法：

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());
第二种：方法labmda语法：

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"
        });


实际使用过的例子：使用的是方法+labmda 

[ChildActionOnly]
public ActionResult IndexCourseOnClickRange()
{
var courseList =new List<IndexClickRangeOnCourseViewModel>();  //先建立一个list .
var _coursesByClick = _resourceService.FindAll().Where(r => r.ResourceIsShow == true) //也可以使用ContextFactory.GetCurrentContext().Source取得课程资源。
.GroupBy(r => r.CourseID)
.Select(g => new IndexClickRangeOnCourseViewModel
{
CourseID = g.Key,
CourseResourceCount =g.Count(),
CourseResourceTotalClickCount = g.Sum(x =>x.ClickCount) //对分组后的数据 按点点击量汇总。
});

var _courses = _coursesByClick.ToList(); //这里是重点，必须使用tolist方法来立即执行，从数据库中获取值，如果不立即执行，延迟执行将会发生错误。

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>; //排序后变成了IOrder ,为了使用take取前面的序列，必须转换成IEnumerable ;
if(coursesOrder.Count() >=7)
{
coursesOrder =coursesOrder.Take(7);

}

return PartialView("_IndexCourseOnClickRange", coursesOrder);
}