'Register filter passed from API layer.
:param filter_name: Name of the filter passed in the URL
Filter names are validated in validate_filters() method which by
default allows filters based on fields\' names.  Extensions can create
new filter names.  Such names must be registered to particular object
with this method.'

@classmethod
def add_extra_filter_name(cls, filter_name):

    cls.extra_filter_names.add(filter_name)
