.net - Sum and Group By in Linq to Entities -

i'm trying sum of values grouping , i'm stuck. sample structure of table:

what want is:

user name, project name, total work time of project users, grouped user name , project name.

the trick need display 0 or null value if user in project , user doesn't have work time yet.

example result should this:

username  project  total time user1     project1 15 user1     project2 -- user2     project1 10 

how can accomplish this?

this scary select work, once add references (navigational properties)

db.users.selectmany(     x=> x.projects            .select(i=>                   new                   {                      user = x.name,                      project = i.name,                      worktime = (int?)i.userworktimes.sum(t=>t.worktime)                    })     );  

my assumptions are

1. worktime of int type.

2. nav.property projects projectuserworktimes called userworktimes

3. nav.property users projects called projects