How the system maintains menu order

Last updated March 25, 2009. Created by sdboyer on May 7, 2007.
Edited by LeeHunter, ax, chx, add1sun. Log in to edit this page.

Note: this page is highly technical and only needed if you want to change menu.inc itself.

We have seven integer columns each representing a piece of the materialized path (the path leading to the root), p1 .. p7 . This algorithm is Peter Wolanin's implementation of materialized path, heavily exploiting the fact that we know the max depth of the tree.

All the numbers you see here are menu link identifiers, a primary integer key for each menu link (in short: mlid). Note that for queries, the list of "parents" is derived from p1,p2,p3, not stored separately. mlid, plid are stored separately.

Materialized path	mlid	plid	set of parents
5.0.0	5	0	0
5.6.0	6	5	5, 0
7.0.0	7	0	0
7.13.0	13	7	7. 0
7.15.0	15	7	7, 0
7.15.23	23	15	15, 7, 0
7.15.16	16	15	15, 7, 0
7.10.0	10	7	7, 0
7.10.22	23	15	15, 7, 0
12.0.0	12	0	0

to get the tree for mlid = 23, we construct parents from p2, p1, 0

result (showing p1.p2.p3):

We need to sort each subtree by weight and title in PHP, but the alternative is heavily renumbering for each edit. Some sorting in php is an acceptable compromise.

If we ran a SELECT on the whole table to get the full tree:

This makes both inserting new items and reparenting existing items quite simple. For example, if reparented the menu item 5.0.0 (aka, mlid == 5) under 7.0.0, this is how the data would look for the same sample menu items we used in the table above:

Materialized path	mlid	plid	set of parents
7.0.0	7	0	0
7.5.0	5	7	7, 0
7.5.6	6	5	7, 5, 0
7.13.0	13	7	7. 0
7.15.0	15	7	7, 0
7.15.23	23	15	15, 7, 0
7.15.16	16	15	15, 7, 0
7.10.0	10	7	7, 0
7.10.22	23	15	15, 7, 0
12.0.0	12	0	0

Note: plid only changed for one item: mlid == 5. Originally plid == 0 where mlid == 5, but now plid == 7. This is similar to the way book module currently operates.

Thus, reparenting generally involves two steps:

1. Put in the new value in the 'plid' column and p1...p6 columns for the reparented item (involves one UPDATE query)
2. Then, run an update for the materialized path columns for all children of the reparented item to reflect the path that their list of parents (path to root or materialzed path) has changed. In our example case, the query for that looks like this:
   
   SET p1 = 7, p2 = p1, p3 = p2 WHERE p1 = 5 and p2 != 0