wp_nav_menu without menu or theme_location specified

Question

Can someone tell me how the wp_nav_menu functions when no menu or theme_location is specified?

I have multiple sites that use an interior navigation setup that does not specify the menu. It would seem that this results in WordPress searching within all menus until the page is found. For 99% of my sites, this functionality works perfectly.

However, I've noticed if multiple menus contain the same page (and these pages have different children specified to them), wp_nav_menu seems to return the first menu that it discovers which contains the desired page. In certain instances, this is ends up being from a menu that does not contain all of the pages in main navigational hierarchy.

Therefore, I was wondering if there was a way to specify the menu order (i.e. check the main menu first, then the footer, then utility, etc), or if a master menu should be implemented that is never displayed on the site, but is utilized to provide the proper page hierarchy for the site.

Any thoughts on approaches or best practices would be helpful.

For full disclosure, please note that I have extended my wp_nav_menu by utilizing WP Nav Plus, but I do not think that plugin changes my question

Answer 1

I am not confident I am following your description completely, but verbatim from the documentation:

If not given a theme_location parameter, the function displays

• the menu matching the ID, slug, or name given by the menu parameter;
• otherwise, the first non-empty menu;
• otherwise (or if the menu given by menu is empty), output of the function given by the fallback_cb parameter (wp_page_menu(), by default);
• otherwise nothing.

Codex > Function Reference/wp nav menu

The behaviour you are seeing might just be incidental to the what menu is “first”, not following some complicated page–related logic.

As for recommendation — why rely on on something like that in first place? Just make it explicit which menu you need and eliminate any ambiguity.