Got it! Added an onchange for the order line field.You will be able to get a list of tuples. Also added a fix in the web module.Data to the readonly field is saved if write function is called. But we want to change the readonly fields data by the onchange functions. Following is the patch

Index: view_form.js
===================================================================
--- openerp/addons/web/static/src/js/view_form.js   
+++ openerp/addons/web/static/src/js/view_form.js   
@@ -833,11 +833,9 @@
                     // Special case 'id' field, do not save this field
                     // on 'create' : save all non readonly fields
                     // on 'edit' : save non readonly modified fields
-                    if (!f.get("readonly")) {
-                        values[f.name] = f.get_value();
-                    } else {
-                        readonly_values[f.name] = f.get_value();
-                    }
+                   values[f.name] = f.get_value();
+                    if (f.get("readonly"))
+                       readonly_values[f.name] = f.get_value();
                 }
             }
             if (form_invalid) {