Get Distinct Entries Based On Specific Column in Entity Framework

Question

I have a SQLite table that contains every test result we\'ve run, and I\'m looking to write an entity framework query that returns only the most recent test result per project.

Answer 1

Try with a subquery instead of a grouping. Like this:

results = await _context.Results
            .Include(x => x.Project)
            .AsNoTracking() 
            .Where( r => r.Id == _context.Results.Where( rr => rr.ProjectId == r.ProjectID).Max( rr => rr.Id) )
            .ToListAsync();

Answer 2

While this isn't portable, here's how this can be done using SQLite-compatable SQL and Entity Framework:

results = await _context.Results
    .FromSqlRaw("SELECT Results.* FROM (SELECT Id, ProjectId, MAX(Updated) as Updated " +
    "FROM Results GROUP BY ProjectId) as latest_results " +
    "INNER JOIN Results ON Results.Id = latest_results.Id")
    .Include(x => x.Project) //not required for question but useful
    .OrderBy(x => x.Project.Name)
    .AsNoTracking()
    .ToListAsync();

If someone has a way to do this in pure LINQ/EF, but still perform the query server-side, I'll happily mark that as the answer, since this is dependent on the exact SQL dialect used.

Answer 3

Your method couldn't be translated to T-SQL, Linq to Entities couldn't recognize it. You can modify the code as below (adding AsEnumerable after AsNoTracking):

.Include(x => x.Project)
.AsNoTracking()
.AsEnumerable()

With AsEnumerable after data is loaded, any further operation is performed using Linq to Objects, on the data already in memory.