Rails 4: Strong parameters: allow hashes

monkey_patch_rails_parameters.rb

module ActionController
  class Parameters
    # See https://github.com/rails/rails/blob/v4.2.5.1/actionpack/lib/action_controller/metal/strong_parameters.rb#L562
    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)

              # Allow Hash
              if [Hash, [Hash]].include?(filter[key])
                element.permit(*Array.wrap(element.keys))
              else
                element.permit(*Array.wrap(filter[key]))
              end
            end
          end
        end
      end
    end
  end
end

# https://github.com/rails/rails/blob/v4.2.5.1/actionpack/lib/action_controller/metal/strong_parameters.rb
params = ActionController::Parameters.new(
  form: {
    scalar: 'foobar',
    many: [
      {
        field1: 'foo'
      },
      {
        fiedl2: 'bar'
      }
    ],
    single: {
      filed3: 'baz'
    }
  }
)

params.require(:form).permit(:scalar, many: Hash, single: Hash)