[wp-hackers] Menu schema

[John Blackbourn]

Just starting this thread to try and move some conversation off of
ticket #11817 (Better Site Menu Management).

It's been pointed out that the schema of the new menu management
functionality might not be optimal.

An issue brought up by ceenz is that menu items are stored in the
posts table (with a post_type of 'nav_menu_item'). This introduces the
following problems (issues by ceenz and myself):

 * Menu items contain very little data that relates to posts and
therefore many fields are not relevant (for example, comment_status
when a menu item relates to a taxonomy term).
 * This introduces data duplication. The post title (or taxonomy
title, object title, etc), post content, comment status, post status,
author, parent relationship, comment count, etc etc, are all
duplicated. Although I've not dug into the code, it also appears that
in the case where a nav menu item relates to a post or a page, some of
these fields (comment_status and post_author at least) don't actually
contain the correct values relating to the post or page. Whether this
is a WIP or not I'm not sure.
 * There is no field in this table to simply relate to the
corresponding object (this is why all our data is duplicated).  *
Because the post_status field is used as a nav menu item descriptor,
this field does not relate to the object that the menu item links to.
 * We now have two places where external links are stored - the links
table and the posts table.

My proposed solution is that menus should instead be treated as a
taxonomy (in fact nacin has already stated [1] that menus are a
taxonomy). We could introcude a taxonomy called 'menu' and each menu
could be a taxonomy term (menu 1, menu 2, etc).

Each menu item would be a term relationship. This handles any menu
item that is in the posts table. For any menu item that is an external
link, we could either create an entry in the posts table like we do
with the current menu system or think about using the links table
instead (but this might not sit well with themes that print out every
link in the links table).

For menu items that are a taxonomy term (eg. category X), this will
need a little work. Simply using the object_id field in the
taxonomy_term_relationships table would introduce ambiguity as an
object in this table only relates to one object type by default
(whichever object_type is specified when the taxonomy is registered
(see register_taxonomy()). I propose that we introduce a new field in
the taxonomy_term_relationships table called 'object_type' and this is
used to identify which object type it relates to.

By treating menus as taxonomies, we retain a correct relational
structure between menus and the objects in them. As a bonus, there is
a (currently unused) term_order field in the
taxonomy_term_relationships table which could be used to describe the
order of items in our menu (this would even work well with nested
menus as the term_taxonomy table has a parent field).

Discuss please!

John

[1] http://core.trac.wordpress.org/ticket/11817#comment:334