The origin statement "which could be ... or whatever" **misguides** many newbies like me.
In fact, the ``login`` function in ``contrib.auth`` stores ``user.pk`` in session, then ``get_user`` function in ``contrib.auth`` gets ``user.pk`` in session and then passes it to your custom ``get_user`` as ``user_id``.
Which means, ``user_id`` prarameter in your custom ``get_user`` has to be the primary key of ``User`` object, too.
This also updates all dependent functionality, including modelform_factory
and modelformset_factory, and the generic views `ModelFormMixin`,
`CreateView` and `UpdateView` which gain a new `fields` attribute.
The auth doc was a single page which had grown unwieldy.
This refactor split and grouped the content into sub-topics.
Additional corrections and cleanups were made along the way.
