<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_entity_id(field_name, attrs):
 """Find the ID for a one to one relationship. The server may return JSON data in the following forms for a :class:`nailgun.entity_fields.OneToOneField`:: 'user': None 'user': {'name': 'Alice Hayes', 'login': 'ahayes', 'id': 1} 'user_id': 1 'user_id': None Search ``attrs`` for a one to one ``field_name`` and return its ID. :param field_name: A string. The name of a field. :param attrs: A dict. A JSON payload as returned from a server. :returns: Either an entity ID or None. """

    field_name_id = field_name + '_id'
    if field_name in attrs:
        if attrs[field_name] is None:
            return None
        elif 'id' in attrs[field_name]:
            return attrs[field_name]['id']
    if field_name_id in attrs:
        return attrs[field_name_id]
    else:
        raise MissingValueError(
            'Cannot find a value for the "{0}" field. Searched for keys named '
            '{1}, but available keys are {2}.'
            .format(field_name, (field_name, field_name_id), attrs.keys())
        )