LINQ加入顺序，然后结果顺序

Question

I have a LINQ to construct a list of buildings, which has a field consisting of a list of Users:

List<Building> buildings = (from b in db.Buildings
    join u in db.BuildingUsers on b.BuildingId equals u.BuildingId into bUsers
    orderby b.BuildDate descending
    select Building.Create(b, bUsers)).ToList();

The UserName is not always the user I want. What I really want is the first User that was ever entered, which I think is correct to assume it would be the BuildingUser with the lowest UserID. So I do the Order By:

List<Building> buildings = (from b in db.Buildings
    join u in db.BuildingUsers on b.BuildingId equals u.BuildingId into bUsers
    orderby u.UserId ascending
    select Building.Create(b, bUsers)).ToList();

That works great, except now my overall list of buildings is in an awkward order. The bUsers is passed into the Create method in the correct order, and I handle the FirstOrDefault() to get the first user and handle the rest. I want the end-result of the overall building list to be in order by BuildDate.

List<Building> buildings = (from b in db.Buildings
    join u in db.BuildingUsers on b.BuildingId equals u.BuildingId into bUsers
    orderby u.UserId ascending, b.BuildDate descending
    select Building.Create(b, bUsers)).ToList();

Of course this doesn't work, because now it first sorts by UserId, then by BuildDate and doesn't quite turn out correctly.

So I want one orderby for the Join, and a different orderby for the main result. I'd rather not split them into multiple methods so I don't have to alter the way I construct my building objects.

Answer 1

You were right in using orderby b.BuildDate descending because that is the ordering expected in your output collection buildings . The ordering of the subcollection bUsers should be performed when passing it to Building.Create() :

List<Building> buildings = (from b in db.Buildings
                         join u in db.BuildingUsers on b.BuildingId equals u.BuildingId into bUsers
                         orderby b.BuildDate descending
                         select Building.Create(b, bUsers.OrderBy(bu => bu.UserId))).ToList();

Based on what you are saying above I might suggest changing Building.Create to accept a single user, in which case you could perform the FirstOrDefault() in this query:

List<Building> buildings = (from b in db.Buildings
                         join u in db.BuildingUsers on b.BuildingId equals u.BuildingId into bUsers
                         orderby b.BuildDate descending
                         select Building.Create(b, bUsers.OrderBy(bu => bu.UserId).FirstOrDefault())).ToList();