docs: Document version states (#362)

* Remove remove_published_where

* Fix language issue with preview links

* Add setuptools to requirements for

* Add explanation of version states.

* Add diagram

* Revert unwanted changes

* Revert changes to test requirements

docs/basic_concepts.rst

@@ -0,0 +1,57 @@
+Introduction
+============
+
+djangocms-versioning is a general purpose package that manages versions
+for page contents and other models within four categories: **published**,
+**draft**, **unpublished**, or **archived**, called "version states".
+
+
+Version states
+--------------
+
+Each versioned object carries a version number, creation date, modification date, a reference to the user who created the version, and **version state**. The states are:
+
+  * **draft**: This is the version which currently can be edited. Only draft versions can
+    be edited and only one draft version per language is allowed. Changes made to draft
+    pages are not visible to the public.
+  * **published**: This is the version currently visible on the website to the public. Only
+    one version per language can be public. It cannot be changed. If it needs to be changed
+    a new draft is created based on a published page and the published page stays unchanged.
+  * **unpublished**: This is a version which was published at one time but now is not
+    visible to the public any more. There can be many unpublished versions.
+  * **archived**: This is a version which has not been published and therefore has never been
+    visible to the public. It represents a state which is intended to be used for
+    later work (by reverting it to a draft state).
+
+Each new draft version will generate a new version number.
+
+.. image:: /static/version-states.png
+     :align: center
+     :alt: Version states
+
+When an object is published, it changes state to **published** and thereby becomes publicly visible. All other version states are invisible to the public.
+
+Effect on the model's manager
+-----------------------------
+
+When handling versioned models in code, you'll generally only "see" published objects:
+
+.. code-block::
+
+    MyModel.objects.filter(language="en")   # get all published English objects of MyModel
+
+will return a queryset with published objects only. This is to ensure that no draft or unpublished versions leak or become visible to the public.
+
+Since often draft contents are the ones you interact with in the admin interface, or in draft mode in the CMS frontend, djangocms-versioning introduces an additional model manager for the versioned models **which may only be used on admin sites and admin forms**::
+
+    MyModel.admin_manager.filter(language="en")
+
+will retrieve all objects of all versions. Alternativley, to get the current draft version you can to filter the ``Version`` object::
+
+    from djangocms_versioning.constants import DRAFT
+
+    MyModel.admin_manager.filter(language="en", versions__status==DRAFT)
+
+Finally, there are instance where you want to access the "current" version of a page. This is either the current draft version or - there is no draft - the published version. You can easily achieve this by using::
+
+    MyModel.admin_manager.filter(language="en").current_content()

docs/index.rst

@@ -5,6 +5,7 @@ Welcome to "djangocms-versioning"'s documentation!
    :maxdepth: 2
    :caption: Quick Start:
 
+   basic_concepts
    versioning_integration
    version_locking