EF4 and UnitOfWork - How to make custom properties work

Say I have:

public class A
{
    public virtual int Id { get; set; }
}

public class ARepository
{
    private SomeContext _context;
    public ARepository(IUnitOfWork unitOfWork)
    {
        _context = unitOfWork;
    }
    public A GetAById(int aId)
    {
        return _context.A.Where(o => o.Id == aId).SingleOrDefault();
    }
}

public class B
{
    public virtual int Id { get; set; }
    public virtual Category Category { get ; set; } //enum with N开发者_如何学GoEW and OLD values
    public virtual int Quantity { get; set; }
}


public class BRepository
{
    private SomeContext _context;
    public BRepository(IUnitOfWork unitOfWork)
    {
       _context = unitOfWork;
    }
    public B GetBById(int bId)
    {
        return _context.B.Where(o => o.Id == bId).SingleOrDefault();
    }
}

And I'm working with Entity Framework (4.1 code first).

How would I implement for example a custom property in A class like:

//returns the total of all B objects in the context where the category is NEW
public int NewBTotals
{
}

And not having to create a context?

Hopefully my question is clear enough, if not please let me know and I'll try to be more descriptive of what I want to achieve (ran out of time).

I found that even by B not having a foreign key to A, I can create a navigation property from A to B and vice versa which kind of solves the problem on its own.