Populating a list within a list in C# not using foreach loop. better way?

Question

I have a list of objects within a list of objects (List-ParentClass) that has as one of its objects a nested list (List-ChildClass). To populate List-ChildClass I have used a foreach loop as shown below. I have also nested a linq query as show below.

At this point I am having some performance issues and I feel like there is a better way to do this that I am just not finding.

Question: How could I do this better/faster?

Note - This is a Web based .net MVC application written in C#. I use EF back to a SQL database.

public class ParentClass
{
    public int pcid { get; set; }
    public List<ChildClass> ChildClassList { get; set; }
}

public class ChildClass
{
    public int pcid { get; set; }
    public int ccid { get; set; }
}

public class DoWork 
{
    public void ExampleMethodForEach()
    {
        List<ParentClass> ParentClassList = new List<ParentClass>();

        foreach(ParentClass a in ParentClassList)
        {
            a.ChildClassList = EFDatabase2.where(b => b.pcid == a.pcid).select(b => b.ccid).ToList();
        }
    }

    public void ExampleMethodLinq()
    {
        var ParentClassList = (from a in EFDatabase
                               select new ParentClass
                               {
                                   ccid = a.ccid,
                                   pcid = (from b in EFDatabase2
                                           where b.pcid == a.pcid
                                           select b.ccid).ToList()
                                   //something like this were I nest a query
                               }).ToList();
    }
}

Answer 1

The best way when working with relational databases and LINQ is to use joins to correlate data. In your case, the most appropriate is group join:

var ParentClassList =
    (from p in EFDatabase
     join c in EFDatabase2 on p.pcid equals c.pcid into children
     select new ParentClass
     {
         pcid = p.pcid,
         ChildClassList =
             (from c in children
              select new ChildClass
              {
                  pcid = c.pcid,
                  ccid = c.ccid
              }).ToList()
     }).ToList();

which should give you a nice fast single database query.

P.S. Hope your EFDatabase and EFDatabase2 variables refer to two tables inside one and the same database.