I am trying to show a list of all tasks for the same customer in a tree view on each task form.  For instance if Customer1 has 3 tasks, I'd like to show those 3 tasks on each task form, sort of a work history tree view.

My first thought on this was to use many2one relation then one2many back but that gives an error about undefined field.  What would be the best method to do this as I could see it being useful in many areas where you want to show all of the objects with a certain field in common on a form view.