Hi,I'm beginner in Openerp. :)

I'm having some problems regarding many2one fields in openerp 6.1.

I added table A with a many2one column that referenced to table B. openerp added foreign key constraint to table A.

but why can I still delete data from table B even if it's still referenced by table A ?

P.S : to disable the deletion I need to drop the constraint manually and add the constraint again.but the constraint revert when I upgrade the module. I want to validate a lot of modules, so I don't want to modify the field one by one. is there any recommended way to do this ? thanks before :)