module JSONAPI::Resources::SortableFieldsDsl

def inherited_sort_only_fields
  parent_sort_fields = superclass.permitted_sortable_fields
  parent_attributes = superclass.permitted_attributes
  parent_sort_fields - parent_attributes
end

def permitted_sortable_fields
  sort_fields = @sortable_fields || []
  sort_fields = inherited_sort_only_fields + sort_fields if should_inherit_sortable_fields?
  (permitted_attributes + sort_fields).uniq
end

def should_inherit_sortable_fields?
  !instance_variable_defined?(:@sortable_fields) &&
    !instance_variable_defined?(:@attributes) &&
    superclass != JSONAPI::Resource &&
    superclass.respond_to?(:permitted_sortable_fields)
end

def sortable_fields(*field_names)
  @sortable_fields ||= []
  @sortable_fields.concat(field_names.map(&:to_sym))
  @sortable_fields.uniq!
end