'Retrieve the External ID of any database record, if there
is one. This method works as a possible implementation
for a function field, to be able to add it to any
model object easily, referencing it as ``Model.get_external_id``.
When multiple External IDs exist for a record, only one
of them is returned (randomly).
:return: map of ids to their fully qualified XML ID,
defaulting to an empty string when there\'s none
(to be usable as a function field),
e.g.::
{ \'id\': \'module.ext_id\',
\'id2\': \'\' }'

@api.multi
def get_external_id(self):

    results = self._get_external_ids()
    return {key: (val[0] if val else '') for (key, val) in results.iteritems()}
