openerp - stop deleting records in one2many relationship in odoo -

i have created relational columns different class when delete relational records delete records base class well.

anyone have solution stop deleting records base class while deleting relational field.

i have checked ondelete='set null' on field declaration.

e.g have create field many2one 'purchase.order', when delete record delete 'purchase.order' well.

i want stop deleting purchase order while delete record.

in order stop field one2many deleting record witch default behavior of field when creating view give option:

<field name="one2many_field_name" options="{'not_delete': true}" />