LINQ/C#：加入、分组并选择每个组中的最后一个？

Question

Using C# and LINQ, I have a table of employees, only some of whom are active and a table of events. I need to join the table of events to the employees on ID, group the events by employee and select the newest event. I've tried things along the line of this but getting crosseyed and nowhere.

 var emp = (from em in dc.Employees
                   where (em.Active == true)
                   join ev in dc.Events on em.ID equals ev.Employee orderby ev.Time

OK, by cannibalizing some older code from another project, I got this:

 var attends = dc.Events               
 .GroupBy(c => c.Employee)
 .Select(x => x.OrderByDescending(y => y.Time).FirstOrDefault())
 .Join(dc.Employees,u => u.Employee,o => o.ID,
        (u, o) => new { o.Name,u.Time,u.EventType}).ToList();

Which seems to work, but now I need to filter for Employees where active is true... where would I do that?

OK... I think (knock on wood) I got it, in the event it helps someone else, this seems to do it:

var attends = dc.Events                
 .GroupBy(c => c.Employee)
 .Select(x => x.OrderByDescending(y => y.Time).FirstOrDefault())
 .Join(dc.Employees.Where (z=>z.Active), u => u.Employee,o => o.ID, 
        (u, o) => new { o.Name,u.Time,u.EventType}).ToList();


    }

If not, I'll be back crying...

Answer 1

Something like this (it maynot compile):

var emp = (from ii in
        (from em in dc.Employees
               where (em.Active == true)
               join ev in dc.Events on em.ID equals ev.Employee 
        select new { Employee = ev.Employee, Event = ev.Eventid, EventTime = ev.Time})
        group ii by new {ii.EventId} into g
        select new 
        {
           Event = g.Key
        }).ToList();

Answer 2

Seems like this works... kind of sad that I got a down vote because apparently someone felt I hadn't done adequate homework. Hopefully this will help someone else:

    var attends = dc.Events                
 .GroupBy(c => c.Employee)
 .Select(x => x.OrderByDescending(y => y.Time).FirstOrDefault())
 .Join(dc.Employees.Where (z=>z.Active), u => u.Employee,o => o.ID, 
        (u, o) => new { o.Name,u.Time,u.EventType}).ToList();

Answer 3

I need to join the table of events to the employees on ID, group the events by employee and select the newest event.

It seems to me that you want a sequence of Employees, each Employee with his newest event.

Usually if you do a Join followed by a GroupBy, you could also use a GroupJoin:

var employeesWithNewestEvents = employees.GroupJoin(events,
    employee => employee.Id,        // from each Employee take the primary key
    event => event.EmployeeId,      // from each event take the foreign key to Employee

    // ResultSelector: from each Employee with his zero or more Events, make a new
    (employee, eventsOfThisEmployee) => new
    {
         // select only the Employee properties you plan to use
        Id = employee.Id,
        Name = employee.Name,
        ...

        NewestEvent = eventsOfThisEmployee
            .OrderByDescending(event => event.Time)
            .Select(event => new
            {
                 // Select only the Event Properties you plan to use
                 Id = event.Id,
                 Description = event.Description,
                 ...

                 // not needed, you know the value: EmployeeId = event.EmployeeId
            })
            .FirstOrDefault(),
    })

Note that Employees without events will also be in the selected data. They will have a null value for NewestEvent.

If you want to select the complete Employee and its complete latest event you could use the following result selector:

(employee, eventsOfThisEmployee) => new
{
     Employee = employee,
     NewestEvent = eventsOfThisEmployee
        .OrderByDescending(event => event.Time)
        .FirstOrDefault(),
});

Be aware that this method will select more properties than you actually need.