[C#] Linq extension: GroupJoin By

///查詢每個月需要用膳的數目

///GroupJoin by CheckInOn月份,

var B = db.Reservations.Where(x => x.IsDinner == true);

var _dinnerInfo = A.GroupJoin( B,

                 x => x, //母體A的join key

                 y => y.CheckInOn.Month, //子體B的join key

                 (x, y) =>

                 new

                 {

                    Month = x,

                    IsDinner = "需要用餐的訂房數",

                    Count = y.Count()

                  });