flectra/addons/event_sale/views/sale_order_views.xml

<?xml version="1.0" encoding="utf-8"?>
<flectra>
        <record id="view_sale_order_form_inherit_event" model="ir.ui.view">
             <field name="name">sale.order.form.inherit</field>
             <field name="model">sale.order</field>
             <field name="inherit_id" ref="sale.view_order_form" />
             <field name="arch" type="xml">
                <xpath expr="//field[@name='order_line']//form//field[@name='product_id']" position="after">
                    <field name="event_id" domain="[('event_ticket_ids.product_id','=', product_id),('date_end','&gt;=',time.strftime('%Y-%m-%d 00:00:00'))]" attrs="{'invisible': [('event_ok', '=', False)],'required': [('event_ok', '!=', False)]}"/>
                    <field name="event_ticket_id" domain="[('event_id', '=', event_id), ('product_id','=',product_id), '|', ('seats_availability', '=', 'unlimited'), ('seats_available', '>', 0)]" attrs="{'invisible': [('event_ok', '=', False)], 'required': [('event_ok', '!=', False)]}"/>
                    <field name="event_ok" invisible="1"/>
                </xpath>
                <xpath expr="//field[@name='order_line']//tree//field[@name='product_id']" position="after">
                    <field name="event_ok" invisible="1"/>
                </xpath>
             </field>
        </record>

        <!-- the presence of 'event user' makes the order lines non-editable -->
        <record id="sale.view_order_form_editable_list" model="ir.ui.view">
            <field name="groups_id" eval="[(4, ref('event.group_event_user'))]"/>
        </record>
</flectra>