hash_filter

# File actionpack/lib/action_controller/metal/strong_parameters.rb, line 560
      def hash_filter(params, filter)
        filter = filter.with_indifferent_access

        # Slicing filters out non-declared keys.
        slice(*filter.keys).each do |key, value|
          next unless value

          if filter[key] == EMPTY_ARRAY
            # Declaration { comment_ids: [] }.
            array_of_permitted_scalars_filter(params, key)
          else
            # Declaration { user: :name } or { user: [:name, :age, { address: ... }] }.
            params[key] = each_element(value) do |element|
              if element.is_a?(Hash)
                element = self.class.new(element) unless element.respond_to?(:permit)
                element.permit(*Array.wrap(filter[key]))
              end
            end
          end
        end
      end