使用多个连接和多个选择字段从 linq 获取最大日期

Question

I have been scratching my head with this and cannot find a solution. I have a dataset like this,

JobID   DueDate     VisitID
4024082 2020-07-31  5267157
4024082 2020-07-29  5263915
4024082 2020-08-04  5269154
4024082 2020-07-23  5258774
4024082 2020-07-22  5255307

I want to get the result set like this,

JobID   Duedate     VisitID
4024082 2020-08-04  5269154

I have tried several ways but cannot get it to work. my linq query is

            var jobs = await (from j in _ctx.Jobs
                              join v in _ctx.Visits on j.JobID equals v.JobID
                              join p in _ctx.Products on j.ProductID equals p.ProductID
                              where j.IsProcessed== true
                              group v by new
                              {
                                  v.JobID,
                                  v.VisitID
                              }
                into g
                              select new
                              {
                                  JobId = g.Key,
                                  DueDate = g.Max(x => x.DueDate),
                                  VisitID = g.Key.VisitID
                              }).ToListAsync();

Any help would be highly appreciated. Thanks

Answer 1

You cannot use Max to get more than one value. You have to use a sorting mechanism here.

The following is an example of how to get the jobs grouped by the JobID and then every highest date with accompanying VisitID .

Here is the example. (I used LinqPad to test this):

void Main()
{
    var jobs = new[]
    {
        new Job { JobID = 4024082, DueDate = DateTime.ParseExact("2020-07-31", "yyyy-MM-dd", CultureInfo.InvariantCulture), VisitID = 5267157 },
        new Job { JobID = 4024082, DueDate = DateTime.ParseExact("2020-07-29", "yyyy-MM-dd", CultureInfo.InvariantCulture), VisitID = 5263915 },
        new Job { JobID = 4024082, DueDate = DateTime.ParseExact("2020-08-04", "yyyy-MM-dd", CultureInfo.InvariantCulture), VisitID = 5269154 },
        new Job { JobID = 4024082, DueDate = DateTime.ParseExact("2020-07-23", "yyyy-MM-dd", CultureInfo.InvariantCulture), VisitID = 5258774 },
        new Job { JobID = 4024082, DueDate = DateTime.ParseExact("2020-07-22", "yyyy-MM-dd", CultureInfo.InvariantCulture), VisitID = 5255307 },
    };
    
    var query = from job in jobs
                group job by job.JobID into grp
                select grp.OrderByDescending(g => g.DueDate).FirstOrDefault();
    
    query.Dump();
}

public class Job
{
    public int JobID { get; set; }
    public DateTime DueDate { get; set; }
    public int VisitID { get; set; }
}

Output:

JobID    DueDate            VisitID
4024082  4-8-2020 00:00:00  5269154 

Answer 2

I think the problem is that you're using VisitId as part of the GroupBy . Since all the VisitId values are unique, you'll get a single record per group (and therefore one group for each record).

Perhaps you should only join on JobId if you want one group from those sample records. Then to get the VisitID for the row that had the most recent DueDate , we can order the group by DueDate (descending) and take the VisitID from the First() record:

var jobs = await (from j in _ctx.Jobs
                  join v in _ctx.Visits on j.JobID equals v.JobID
                  join p in _ctx.Products on j.ProductID equals p.ProductID
                  where j.IsProcessed== true
                  group v by v.JobID
                  into g
                      select new
                      {
                          JobId = g.Key,
                          DueDate = g.Max(x => x.DueDate),
                          VisitID = g.OrderByDecending(x => x.DueDate).First().VisitID
                      }).ToListAsync();