Value of a reference property.

        You can set the reference with a Part, Part id or None value.
        Ensure that the model of the provided part, matches the configured model

        :return: a :class:`Part` or None
        :raises APIError: When unable to find the associated :class:`Part`

        Example
        -------
        Get the wheel reference property

        >>> part = project.part('Bike')
        >>> wheels_ref_property = part.property('Wheels')
        >>> isinstance(wheels_ref_property, MultiReferenceProperty)
        True

        The value returns a list of Parts or is an empty list

        >>> type(wheels_ref_property.value) in (list, tuple)
        True

        Get the selection of wheel instances:

        >>> wheel_choices = wheels_ref_property.choices()

        Choose random wheel from the wheel_choices:

        >>> from random import choice
        >>> wheel_choice_1 = choice(wheel_choices)
        >>> wheel_choice_2 = choice(wheel_choices)

        Set chosen wheel
        1: provide a single wheel:

        >>> wheels_ref_property.value = [wheel_choice_1]

        2: provide multiple wheels:

        >>> wheels_ref_property.value = [wheel_choice_1, wheel_choice_2]

def value(self):
        """Value of a reference property.

        You can set the reference with a Part, Part id or None value.
        Ensure that the model of the provided part, matches the configured model

        :return: a :class:`Part` or None
        :raises APIError: When unable to find the associated :class:`Part`

        Example
        -------
        Get the wheel reference property

        >>> part = project.part('Bike')
        >>> wheels_ref_property = part.property('Wheels')
        >>> isinstance(wheels_ref_property, MultiReferenceProperty)
        True

        The value returns a list of Parts or is an empty list

        >>> type(wheels_ref_property.value) in (list, tuple)
        True

        Get the selection of wheel instances:

        >>> wheel_choices = wheels_ref_property.choices()

        Choose random wheel from the wheel_choices:

        >>> from random import choice
        >>> wheel_choice_1 = choice(wheel_choices)
        >>> wheel_choice_2 = choice(wheel_choices)

        Set chosen wheel
        1: provide a single wheel:

        >>> wheels_ref_property.value = [wheel_choice_1]

        2: provide multiple wheels:

        >>> wheels_ref_property.value = [wheel_choice_1, wheel_choice_2]

        """
        if not self._value:
            return None
        if not self._cached_values and isinstance(self._value, (list, tuple)):
            ids = [v.get('id') for v in self._value]
            self._cached_values = list(self._client.parts(id__in=','.join(ids), category=None))
        return self._cached_values