def load_applied_versions
      database_versions.tap do |versions|
        versions.each{|v| VersionObjectLoader.new(v).load }
      end
    rescue Cassandra::Errors::InvalidError => e
      raise uninitialized_error
    end
def load_applied_versions
      database_versions.tap do |versions|
        versions.each{|v| VersionObjectLoader.new(v).load }
      end
    rescue Cassandra::Errors::InvalidError => e
      raise uninitialized_error
    end
load version migration class from disk