Left join with grouping and ordering - linq

Question

Musicians write songs. Songs are played on the air.
I have database tables Musicians, Songs and AirTimes. The AirTimes table entries hold information on which song was played on which date and for how many minutes.
I have classes Musician, Song, AirTime that correspond to the tables. The classes have navigational properties that point to the other entity. Arrows below represent navigation.
Musician <--> Song <--> AirTime
From the database, I have to retrieve all the Musicians and dates on which his/her song got AirTime. Plus I want to show the number of Songs played on a particular date and the number of minutes played on that date.
In Microsoft SQL, I would do it as follows:
select
dbo.Musicians.LastName
, dbo.AirTimes.PlayDate
, count(dbo.AirTimes.PlayDate) as 'No. of entries'
, sum(dbo.AirTimes.Duration) as 'No. of minutes'
from dbo.Musicians
left outer join dbo.Songs
on dbo.Musicinas.MusicianId = dbo.Songs.MusicianId
left outer join dbo.AirTimes
on dbo.Songs.SongId = dbo.AirTimes.SongId
and '2014-07-01T00:00:00' <= dbo.AirTimes.PlayDate
and dbo.AirTimes.PlayDate <= '2014-07-31T00:00:00'
group by
dbo.Musicians.LastName
, dbo.AirTimes.PlayDate
order by
dbo.Musicians.LastName
, dbo.AirTimes.PlayDate
Can anybody “translate” this into linq-to-entitese?
Update Aug. 9, 2012
I'm unable to confirm grudolf's schemes do what I wanted. I accomplished things with a different technique. Nonetheless, I accept his/her answer.

Answer 1

As you have the navigational properties in both directions you can start either from AirTimes:
var grpTime = (
from a in AirTimes
where a.Date >= firstDate && a.Date < lastDate
group a by new {a.Song.Musician.LastName, a.Song.Title, a.Date} into grp
select new {
grp.Key.LastName,
grp.Key.Title,
grp.Key.Date,
Plays = grp.Count(),
Seconds = grp.Sum(x => x.Duration)
}
);
or from Musicians:
var grpMus = (
from m in Musicians
from s in m.Songs
from p in s.Plays
where p.Date >= firstDate && p.Date < lastDate
group p by new {m.LastName, s.Title, p.Date} into grp
select new {
grp.Key.LastName,
grp.Key.Title,
grp.Key.Date,
Plays = grp.Count(),
Seconds= grp.Sum(x => x.Duration)
}
);
EDIT:
To display all musicians, including those without airtime you can use another level of grouping - in first step you calculate totals per song+day and then group them with song's author. It could probably work directly with database but I didn't manage to find an efficient way to do it. Yet. ;) With code, the original AirTimes result is changed to return Musician instead of his lastname and then joined to list of all musicians:
//Airtimes for musicians
var grpAir = (
from a in AirTimes
where a.Date >= firstDate && a.Date < lastDate
group a by new {a.Song.Musician, a.Date} into grp
select new {
//Musician instead of his LastName for joining. Id would work too
grp.Key.Musician,
//grp.Key.Musician.LastName,
Date=grp.Key.Date,
Plays = grp.Count(),
Secs = grp.Sum(x => x.Duration)
}
);
var res = (
from m in Musicians
join g in grpAir on m equals g.Musician into g2
from g in g2.DefaultIfEmpty()
orderby m.LastName
select new {
m.LastName,
Date = (g==null ? null : g.Date),
Plays = (g==null ? 0 : g.Plays),
Secs = (g==null ? 0 : g.Secs)
}
);
You can find a more complete LINQPad sample at https://gist.github.com/3236238