根据另一个IQueryable的值对IQueryable进行排序

Question

I've got two IQueryables returned from Linq to SQL and I want to sort one by the other. Essentially, this is what I have

public class From
{
    public int ID { get; set; }
    public string Name { get; set; }
}

public class Message
{
    public int FromID { get; set; }
    public string Subject { get; set; }
    public DateTime SentDate { get; set; }
}

public class MetaMsg
{        
    public string FromName { get; set; }        
    public string Subject { get; set; }
    public DateTime SentDate { get; set; }
}

I want to return a list of 10 Message objects sorted by either From.Name , Subject or SentDate

The only way I can do this right now is to return all Message and From objects from the database and manually combine them in to a meta object, like so

List<MetaMsg> list = new List<MetaMsg>();
var froms = db.From.Where(//etc).ToList();
var messages = db.Message.Where(//bla bla).ToList();

foreach(Message m in messages)
{
    MetaMsg mm = new MetaMsg {
        Subject = m.Subject, 
        SentDate = m.SentDate
        FromName = froms.Where(f = f.ID == m.FromID).FirstOrDefault().Name
    };
    list.Add(mm);
}

if(//sort by subject)
{
    list = list.OrderBy(x => x.Subject).Take(10).ToList();        
}
else if(//sort by date)
{
    list = list.OrderBy(x => x.SentDate).Take(10).ToList();        
}
else
{
    list = list.OrderBy(x => x.FromName).Take(10).ToList();
}

Is there a way to do this without returning a full list of Message and From first?

UPDATE

Here's a better explanation of what I want to do in franken-code (that's not quite pseudo-code, not quite real-code)

List<MetaMsg> list = new List<MetaMsg>();

// get a list of 'From's, ordered by name
var froms = db.From.Where(//etc).OrderBy(x => x.Name);

var messages = null;

if(//sort by subject)
{
    // order by subject and take 10
    messages = db.Messages.OrderBy(x => x.Subject).Take(10).ToList();        
}
else if(//sort by date)
{        
   // order by date and take 10
    messages = db.Messages.OrderBy(x => x.Subject).Take(10).ToList();      
}
else // sort by from Name...
{
    // try and select 10 successive messages which have the fromID 
    // of our first (sorted) from on the list

    int i = 0;        
    while(messages.Count < 10)
    {
        // if i is greater than the number of items in from, stop!
        if(i > from.Count)
            break;

        messages += db.Messages.Where(x => x.FromID == from[i].ID).Take(10).ToList();

        // if we don't quite make 10, keep going with the next id
        i++;
    }
}

// now create our list of froms. 
foreach(Message m in messages)
{
    MetaMsg mm = new MetaMsg {
        Subject = m.Subject, 
        SentDate = m.SentDate
        FromName = froms.Where(f = f.ID == m.FromID).FirstOrDefault().Name
    };
    list.Add(mm);
}

The subtle difference here is that this second example only ever selects 10 message objects, where as the first example has to select all of them in order to sort by from name.

Technically the code in my second example should work (I haven't tried it) but I'm wondering if there's a more elegant way of doing this?

Answer 1

You can directly project to MetaMsg like this:

var result = (from m in db.Messages
              let fromName = (from f in db.From
                              where m.FromId == f.ID                                  
                              select f.Name).FirstOrDefault()
              /* If you have navigation properties setup, you may be able to do this */
              let fromName = m.From.Name
              orderby fromName 
              select new MetaMsg
              {
                  Subject = m.Subject, 
                  SentDate = m.SentDate,
                  FromName = fromName,                      
              }).Take(10).ToList();