Selecting single element from each group that cointains maximum DateTime value

Question

model :

public class ReferenceParameterHistory
{
    [Key]
    public int IDReferenceParameterHistory { get; set; }
    public double Value { get; set; }
    public string Value_S { get; set; }
    public DateTimeOffset CreatedAt { get; set; }
    [Required]
    public bool IsStable { get; set; }

    public int? IDReference { get; set; }
    public Reference Reference { get; set; }

    [Required]
    public int IDParameterTemplate { get; set; }
    public ParameterTemplate ParameterTemplate { get; set; }
}

My code in ASP.NET core controller :

            [HttpGet]
    public async Task<IActionResult> GetReferenceParameterHistory(int? IDparameterTemplate,
                                                                  int? IDreference,
                                                                  DateTimeOffset? startDate,
                                                                  DateTimeOffset? endDate,
                                                                  bool latestOnly)
    {
        try
        {
            IQueryable<ReferenceParameterHistory> query = _context.ReferenceParameterHistory.OrderByDescending(rph => rph.IDReferenceParameterHistory);
            if (IDparameterTemplate != null && IDparameterTemplate > 0)
                query = query.Where(rph => rph.IDParameterTemplate == IDparameterTemplate);
            if (IDreference != null && IDreference > 0)
                query = query.Where(rph => rph.IDReference == IDreference);
            if (startDate != null)
                query = query.Where(rph=> rph.CreatedAt >= startDate);
            if (endDate != null)
                query = query.Where(rph => rph.CreatedAt <= endDate);
            if (latestOnly)
            {
                // I tried this but it doesnt compile and I don't have idea how to solve this ....

                //query = (from rph in query
                //         group rph by rph.IDParameterTemplate
                //            into groups
                //            where groups.Max(rph => rph.CreatedAt)
                //         select groups.Key);
            }


            var referenceParameterHistory = await query.AsNoTracking().ToListAsync();
            if (referenceParameterHistory.Any())
                return new ObjectResult(referenceParameterHistory);

            return new NotFoundResult();
        }
        catch (Exception ex)
        {
            _logger.LogError(ex.Message);
            return new StatusCodeResult(500);
        }
    }

I have a database table based on that ReferenceParameterHistory model class. I want to group records exctracted from that table by IDParameterTemplate and from each group I need to extract records that have the highest value in CreatedAt column (latest records). So each group contains many recods but I need to get only these with max value in CreatedAt column. The result should be IEnumerable of ReferenceParameterHistory since I store that query in an IQueryable variable and then send it to SQL Server to process the query. Commented code in my example is just what I tried but I don't know how to do that.

How can I solve that problem ?

score 0 · Answer 1 · answered May 17 '18 at 23:44

You reused the variable rph inside the lambda.

How to select only the records with the highest date in LINQ

query = (from rph in query
group rph by rph.IDParameterTemplate into g
select g.OrderByDescending(t=>t.CreatedAt).FirstOrDefault());

Selecting single element from each group that cointains maximum DateTime value

1 Answers1