fetch_value

# File activemodel/lib/active_model/attribute_set/builder.rb, line 41
    def fetch_value(name, &block)
      if attr = @attributes[name]
        return attr.value(&block)
      end

      @casted_values.fetch(name) do
        value_present = true
        value = values.fetch(name) { value_present = false }

        if value_present
          type = additional_types.fetch(name, types[name])
          @casted_values[name] = type.deserialize(value)
        else
          attr = default_attribute(name, value_present, value)
          attr.value(&block)
        end
      end
    end