Improvement: Allow Join with DB.UpdateAndGet()

[mulder999]

I would like to atomically set a business lock on a child entity if its parent matches a given condition.

This would likely requires the use of Join operator which is unfortunately not available with DB.UpdateAndGet().

Basically, I desire to achieve something like the pseudo code below:

var child = DB.UpdateAndGet<Person>()
  .Match(x => x.Parent.Age < 30) // Not working with Reference
  .Modify(x => x.IsLocked, true);

public class Person
{
  public bool IsLocked { get; set; }
  public int Age { get; set; }
  public One<Person> Parent { get; set; }
}

REM: Sort is also lacking from UpdateAndGet()