odoo chatter message on demand

def _message_order_formatter(subj, line, changed, old):
    """Format mail msg for update order lines log.

    @param subj: subject of the message
    @param line: order line
    @param changed: mapping of changed field values
    @param old: mapping of old field values
    """
    mess = subj + "<ul>"
    for fname, value in changed.iteritems():
        mess += """
        <li>{label}: {old_value} -> {new_value}</li>
        """.format(
            # TODO: translate label
            label=line._fields[fname].string,
            old_value=old[fname],
            new_value=value,
        )
    mess += "<ul>"
    return mess


[...]
fnames = (
    'name', 'product_id',
    'product_uom', 'product_uom_qty',
)
changed = {}
current = self.order_line_id.read(
    fnames, load='_classic_write')[0]
for fname in fnames:
    if not current[fname] == line_values[fname]:
        # build mapping of changed field
        # values are ($old, $new)
        changed[fname] = line_values[fname]
if changed:
    self.order_line_id.write(changed)
    subj = 'Line {} updated'.format(self.order_line_id.name)
    self.order_id.message_post(
        body=_message_order_formatter(
            subj,
            self.order_line_id,
            changed,
            current
        ),
    )