Question

I’ve found several posts detailing how to perform a weighted average based on a foreign key, but I have yet to find a solution that deals with my situation. Here it is:

I have two tables, table A and a table B many-to-many table linking them; nothing complicated:

TableA
{
     A_ID,
     Other stuff
}

TableB
{
     B_ID,
     Date
     Other stuff
}

LinkAtoB
{
     A_ID,
     B_ID
}

Now here comes the math part. I’m more or less trying to weight result from TableA based on the number of recent associations in Table B.

So if TableA has 4 associations in table with the following dates:

{10/23/2010, //3 days ago
10/19/2010,  //5 days ago
10/18/2010,  //6 days ago
9/13/2010}   //40ish days ago

So here is how I’d like to rank them:

I’d like to provide a recency threshold in days, I’ll use 7 days as an example:

So using the above data I would assign the following values:

{10/23/2010, //7-3 = 4
10/19/2010,  //7-5 = 2
10/18/2010,  //7-6 = 1
9/13/2010}   //40ish days ago

So the value of the weighted average for that particular TableA entry is then 7 / 3 = 2.33333.

Here is more or less what I have so far:

var k = from a in TableA
        group a by a.Select(x=>x.LinkAtoB.TableB)
                    .Where(x=>x.Date.CompareTo(DateTime.Now.AddDays(-7)) >= 0)
                    into g
        select g.Sum(x => DateTime.Now.Subtract(x.Date).Days) / 
        g.Sum(x => x.Length);

I think I’m close but I know I have the group part wrong. I think the other stuff should work. How do I fix my code to accomplish what I want?

Answer 1

Here you go! 🙂

var k = (from b in TableB
        join bb in LinkAtoB on b.B_ID equals bb.B_ID into b_join
        from ab in b_join.DefaultIfEmpty()
        where b.B_DATE.CompareTo(DateTime.Now.AddDays(-7)) > 0
        select new {ab.A_ID, DaysAgo = (DateTime.Now - b.B_DATE).Days} into xx
        group xx by xx.A_ID into yy
        select new {yy.Key, Weighted = yy.Sum(x=> 7 - x.DaysAgo) / yy.Count()} into zz
        join a in TableA on zz.Key equals a.A_ID
        select new {a.A_ID, a.A_Other_Stuff, zz.Weighted}).ToList();