make it a functional field, the value of that type of field is calculated on the fly, you give it a function name in your class and the return value of that function is the field value (it actually has to return a dictionary of values so that it works in tree views as well as form views) there is some documentation on them here: https://doc.openerp.com/v6.0/developer/2_5_Objects_Fields_Methods/field_type.html/#functional-fields the main non-obvious trick is that the function definition has to be above the field definition (before the _columns={} bit)

Check example of Functional Field. link text