Schema field's default value depending on another field's value select

1

I have 2 fields.First a Choice field called user . i have another List field called roles.
On select of the user in the first field, I want the the default value be changed in the roles field.

How to change the default value of one field by selecting value in another field?
How to achieve this?

i have code below.But here for all user entries same default value only ie Tester. Now i want as
(i)for user aaa role Tester,
(ii)for user bbb role Writer,
(iii)for user ccc role designer

http://pastebin.com/ustiZT8q

2

Have a look at this addon: https://pypi.python.org/pypi/plone.formwidget.masterselect/