database_helper.rb

module DatabaseHelper
  def next_id(obj)
    table_name = obj.class.table_name
    seq = ActiveRecord::Base.connection.default_sequence_name(table_name)
    ActiveRecord::Base.connection.select_value("SELECT last_value FROM #{seq}").to_i + 1
  end
end

RSpec.configure do |config|
  config.include DatabaseHelper
end