Refs #14030 -- Updated docs per "Improved expression support for python values."

    :class:`~django.db.models.Model` has a field named ``then``. This can be

    resolved in two ways::

        >>> from django.db.models import Value

        >>> When(then__exact=0, then=1)

        >>> When(Q(then=0), then=1)

``Case()`` accepts any number of ``When()`` objects as individual arguments.

Other options are provided using keyword arguments. If none of the conditions

evaluate to ``TRUE``, then the expression given with the ``default`` keyword

argument is returned. If no ``default`` argument is provided, ``Value(None)``

is used.

argument is returned. If a ``default`` argument isn't provided, ``None`` is

used.

If we wanted to change our previous query to get the discount based on how long

the ``Client`` has been with us, we could do so using lookups::