Question

Using EF4 and Linq

I have a data structure which looks like this:

 parentid   childid  version
        1         2        1
        1         3        1
        1         4        1
        1         2        2
        1         3        2
        1         5        2
     ...

That is, I want to select the parentid, childid but only for the highest version, for every parentid there is in the database.

My first attempt was this:

var links = data
           .GroupBy (link => link.parentid)
           .Select(ig => ig.OrderByDescending(link => link.version).First())
           .Select ( link => new ....... );

However, this obviously only selects one of the child id’s for each parent id..

In the sample data above I want to get the child ids 2,3,5 from version 2 for parent 1 that is..

Answer 1

I ended up using a subquery like this:

var links = data
    .OrderBy(link => link.parentid)
    .ThenBy(link => link.childid)
    .Where(o => o.version == data
            .Where(i => i.version == o.version).Max(l => l.version))
    .Select(link => new ....);

This way, I get all the parentid’s in the db with only the children with the max version for that specific parent.