[FIX] point_of_sale: session_id field should always be set

Move required attribute to field instead of view. The orders created from the
pos should also have a specified session.

addons/point_of_sale/point_of_sale.py

@@ -706,7 +706,7 @@ class pos_order(osv.osv):
         'sequence_number': fields.integer('Sequence Number', help='A session-unique sequence number for the order'),
 
         'session_id' : fields.many2one('pos.session', 'Session', 
-                                        #required=True,
+                                        required=True,
                                         select=1,
                                         domain="[('state', '=', 'opened')]",
                                         states={'draft' : [('readonly', False)]},

addons/point_of_sale/point_of_sale_view.xml

@@ -26,7 +26,7 @@
                     <group col="4" colspan="4" name="order_fields">
                         <field name="name"/>
                         <field name="date_order"/>
-                        <field name="session_id" required="1"/>
+                        <field name="session_id" />
                         <field name="partner_id" on_change="onchange_partner_id(partner_id, context)" domain="[('customer', '=', True)]" context="{'search_default_customer':1}" attrs="{'readonly': [('state','=','invoiced')]}"/>
                     </group>
                     <notebook colspan="4">