knowledge/document_choose_directory/view/document_directory.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <record id="view_document_directory_form" model="ir.ui.view">
            <field name="model">document.directory</field>
            <field name="inherit_id" ref="document.view_document_directory_form" />
            <field name="arch" type="xml">
                <xpath expr="//page[@string='Definition']/group[2]" position="attributes">
                    <attribute name="attrs" />
                </xpath>
                <xpath expr="//page[@string='Definition']/group[2]/field[@name='resource_field']" position="attributes">
                    <attribute name="attrs">{'invisible': [('type', '!=', 'ressource')]}</attribute>
                </xpath>
                <xpath expr="//page[@string='Definition']/group[2]/field[@name='ressource_tree']" position="attributes">
                    <attribute name="attrs">{'invisible': [('type', '!=', 'ressource')]}</attribute>
                </xpath>
            </field>
        </record>
    </data>
</openerp>