Linq 未分组

Question

Given the data below, I am trying to write a LINQ statement that will group by ParentProductId , and select the maximum EndDate if there is more than one item in the group.

Since the data listed below has two items with the same ParentProductId , I would expect three records to be returned (and for the "2020" date to be used for the ParentProductId = 1 group, not "2019"). However, the LINQ statement that I have is still returning all four records. What am I doing wrong?

Data:

Subscription.Add(new Subscription() { CustomerId = 555, ParentProductId = 37 , EndDate= null});
Subscription.Add(new Subscription() { CustomerId = 555, ParentProductId = 38 , EndDate = null });
Subscription.Add(new Subscription() { CustomerId = 555, ParentProductId = 1  , EndDate = new DateTime(2019, 11, 28) });
Subscription.Add(new Subscription() { CustomerId = 555, ParentProductId = 1, EndDate = new DateTime(2020, 1, 28) });

LINQ Statement:

var IndividualSubscription = (from s in db.Subscriptions
                              join ptp in db.ProductToProducts on s.ProductToProductId equals ptp.ProductToProductId
                              join p in db.Products on ptp.ParentProductId equals p.ProductId
                              where SubscriptionIds.Contains(s.OriginalSubscriptionId)
                              && s.CustomerId == CustomerId
                              group new  {ptp.ParentProductId, s.EndDate }
                              by new 
                              {
                              s.CustomerId,
                              ptp.ParentProductId,
                              p.Name,
                              s.EndDate,
                              } into grp
                              select new NCCN.Model.IndividualGroupSubscription
                                {
                                    CustomerId = grp.Key.CustomerId,
                                    ParentProductId = grp.Key.ParentProductId,
                                    ParentProductName = grp.Key.Name,
                                    EndDate = grp.Max(p => p.EndDate),
                                }).ToList();

Answer 1

Note sure why your linq is that overkill but following your written requirement and not your code what you need is a million times shorter than what you have wrote

// get grouped by product id
var result = Subscription.GroupBy(s => s.ParentProductId) 

// select to output something for each group. What you want is
// order descending all elements and pick the first which is the highest

                         .Select(grp => grp.OrderByDescending(o => o.EndDate).FirstOrDefault())

// returned as a list

                         .ToList();

Answer 2

You group for {ptp.ParentProductId, s.EndDate } .

Group only for ParentProductId, if you need 3 rows result.

var subscription = new List<Subscription>(){new Subscription() { CustomerId = 555, ParentProductId = 37 , EndDate= null}
    ,new Subscription() { CustomerId = 555, ParentProductId = 38 , EndDate = null }
    ,new Subscription() { CustomerId = 555, ParentProductId = 1  , EndDate = new DateTime(2019, 11, 28) }
    ,new Subscription() { CustomerId = 555, ParentProductId = 1, EndDate = new DateTime(2020, 1, 28) }
};

var query = subscription
    .GroupBy(x=> x.ParentProductId)
    .Select(x=> new {x.Key, EndDate = x.Max(s=>s.EndDate)});