destruct_jsonapi.rb

class ActionController::Parameters
  def destruct_jsonapi
    data          = fetch(:data)
    relationships = data.fetch(:relationships) { self.class.new }

    attributes = self.class[*data.fetch(:attributes) { Hash.new }.flat_map {|key, value|
      [key.underscore, value]
    }]

    relationships.each_with_object(attributes) {|(key, value), attrs|
      k = key.underscore

      case _data = value.fetch(:data)
      when Array
        attrs["#{k}_ids"] = _data.map {|item|
          item.fetch(:id)
        }
      else
        attrs["#{k}_id"] = _data.try(:fetch, :id)
      end
    }
  end
end

ActionController::Parameters.new(
  data: {
    attributes: {
      'x-y': 1,
      z:     2
    },
    relationships: {
      'foo-bar': {
        data: {
          id: 42
        }
      },
      baz: {
        data: nil
      },
      qux: {
        data: [{
          id: 3
        }, {
          id: 4
        }]
      }
    }
  }
).destruct_jsonapi.permit(:x_y, :z, :foo_bar_id, :baz_id, :qux_ids => [])
#=> {"x_y"=>1, "z"=>2, "foo_bar_id"=>42, "baz_id"=>nil, "qux_ids"=>[3, 4]}