Linq：按一列分组但必须与另一列不同

Question

In my linq query I want to query the service table which has a visitId and a deviceId. I want to be able to group by the visit but have the deviceId distinct. Currently with my setup I can group by visitId but I'm having trouble showing disitnct devices in the result when the visit is the same.

from s in db.Services.Where(condition)
                 where s.Active && (hasSupportGeneralAccess || s.Visit.Site.Branch.Users.Any(u => u.Id == userId)) && s.Device != null
                 orderby s.Visit.Start descending
                 group s by s.Visit into newgroup
                 select newgroup.FirstOrDefault()

Any Help is greatly appreciated!

Answer 1

After you have grouped by Visit, all your entities (I'm guessing a Service given that it comes from a collection called Services) are in a collection associated with a given visit. If you want to know the distinct devices in that list you can ask for them:

select newgroup.Select(x => x.Device).Distinct()

newgroup is like a List<Service> , and one of the properties of a Service is Device. If I'd handed you a List<Service> and asked for all the distinct Device in it, you'd Select the Device out of the list and then ask for Distinct.. this is no different except that instead of one List<Service> you've got one list per unique Visit - if there are 10 Visits then there are 10 List<Service> you need to process

Answer 2

In this case, we can group the combination of both visit and device like below so that we get repeated device for the same visit.

from s in db.Services.Where(condition)
where s.Active && (hasSupportGeneralAccess ||  s.Visit.Site.Branch.Users.Any(u => u.Id == userId)) && s.Device != null orderby s.Visit.Start descending
group s by (s.Visit + "." + s.Device) into newgroup
select newgroup.FirstOrDefault();