Solve Linq expression using Distinct, GroupBy and Take - linq

Question

I am using Entity Framework, working with lambda expressions, I have to select number of selected records from the grid (_numRecords) which includes sorting, remove duplicates based on a column (Distinct by a column). Following is the code:
private IEnumerable<ReadViewModel> generateLocalData(IQueryable<ReadViewModel> query, [DataSourceRequest] DataSourceRequest dsRequest, int exportXml)
{
if (exportXml > 0)
query = query.GroupBy(x => x.Id).Select(x => x.FirstOrDefault());
//query = query.OrderByDescending(x => x.EventDate).GroupBy(x => x.Id).Select(x => x.FirstOrDefault());
//query = query.DistinctBy(x => x.Id).AsQueryable<ReadViewModel>();
query = query.OrderByDescending(x => x.EventDate);
query = query.ApplySorting(dsRequest.Groups, dsRequest.Sorts);
query = query.Take(this._numRecords);
List<ReadViewModel> data;
data = query.ToList();
return data;
}
public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
HashSet<TKey> knownKeys = new HashSet<TKey>();
foreach (TSource element in source)
{
if (knownKeys.Add(keySelector(element)))
{
yield return element;
}
}
}
There is data on the grid (which can have duplicate Id field) but when I export it to xml, it should be unique.
Problem is when Distinct is applied to lambda expression it destroys the sorting, and I am using Take() method so I need to use it before fetching data from DB.
I've used 2 approaches to acheive distinct: by GroupBy & Single/Select and DistinctBy from Extension methods. But I'm unable to make it work, using these methods, as sorting is destroyed.

Answer 1

private static IQueryable<T> DistinctBy<T, TKey>(this IQueryable<T> source, Expression<Func<T, TKey>> keySelector)
{
return source.GroupBy(keySelector).Select(grp => grp.First());
}
Group by the key you want to be distinct by, then take the first item from each group. You might need FirstOrDefault() with some providers that don't do well with First() within queries.
This is sub-optimal when used with linq-to-objects, but you have the optimal approach for that case in your question already.