LINQ查询上的两个GroupBy

Question

Aim :

To return an IEnumerable list of Model .

Problem :

I'm trying to return a list of my model and group on a substring of a field in my database. However I need to group by twice as the first time I group, the substring has occurred yet and therefore isn't actually grouping on anything as the data is unique before the substring.

Edit :

I do not need the first group by. I am only including it because this was the query before, I am not sure how to do this query. I need to select the nominal period. Get the first four characters of the Nominal Period. Then do a group by on the new 'Nominal Period'

Example of a Nominal Period - 201304

This is made up of the year and then month. Ie todays nominal period would be 201509.

Therefore whenever I return a Nominal Period with just the first four characters, there will be 12 instances of each nominal period as there are 12 months in the year. This is why I need to do the second group by.

Model :

 public class YearsModel {
        public string Year { get; set; }
    }

View Model :

 public class ReportingHubViewModel : ReportViewModel {

        public IEnumerable<YearsModel> Years { get; set; }

        public YearsModel YearsModel { get; set; }

    }

Controller :

model.Years = detailRepo.GetAllYearsFrom(model.ClientID);

Repository :

public class SQLDetailRepository : IDetailRepository {
        ApplicationDbContext camOnlineDb = new ApplicationDbContext();
        public IEnumerable<YearsModel> GetAllYearsFrom(int ClientID) {
            var x = (from d in camOnlineDb.Details
                     where d.ClientID == ClientID
                     where d.NominalPeriod != null

                     select new {
                         d.NominalPeriod
                     });

            return x.GroupBy(r => new { r.NominalPeriod })
                .Select(g => new YearsModel {
                    Year = g.Key.NominalPeriod.ToString().Substring(0, 4)
                })//Need another group by here to group after the substring has occrured 
        }
}

Answer 1

You can safely ignore using GroupBy in this case.

One way is to use Distinct() like following

    return x.Select(y=>.ToString().Substring(0, 4))
            .Distinct()
            .Select(z=>new YearsModel {Year = z})
            .ToList();