[wp-hackers] meta_key and meta_value query variables don't work in the backend?

[Otto]

On Fri, Jan 15, 2010 at 1:24 PM, Matthew Gerring <mgerring at emrl.com> wrote:
> What I mean is, I'm writing a plugin to allow a client to sort pages on the edit pages screen. I have it set up to sort by author, title, and date, and filter by a custom taxonomy. I want to be able to sort by custom field as well- all of the pages in this site have a custom field called "Country" with a set drop-down list of countries.
>
> To do all the sorting and filtering so far, I used the manage_pages_query hook to add query variables to the request, and it has worked perfectly. All it does is put all the query variables in an array and pass it to the wp() function. But if I try to use the meta_key and meta_value variables, which are available if I use them in a theme (i.e. in the front end), nothing happens. So it appears that while you can pull out posts according to custom fields in a theme, you can't do the same thing on the manage pages/posts screens in the back end, and I'm wondering why.
>
> I hope that clears it up.
>
> -Matthew


Ahh. Okay, your use of front/back confused me, as that's backwards
from the way I'd normally think of them.

I'm still not entirely clear on how you're passing this data into the
system, but if it's through the front door (the wp() function), then
that's different than passing it directly to a wp_query.

Passing something into wp() means that it gets parsed as if it came
directly from the untrusted browser, like a URL would. This goes
through all the rewrite rules and that sort of jibber jabber. This
also means that in order for something to make it from the query_vars
there into the wp_query object, then it has to exist in the
public_query_vars.

This is a bit complicated, but basically for some query variables, you
can pass them in on the URL line. Like http://example.com/?p=123 or
http://example.com/?cat=456. That p and cat get converted into
variables inside the wp_query object. Like $wp_query->query_vars['p']
or $wp_query->query_vars['cat']. The wp_query then examines what it
gets and builds the query accordingly.

The meta_key and meta_value are values that the wp_query is capable of
using. But they're not values that the parser is capable of parsing,
by default. So passing them into wp() makes them get filtered out, as
any other unknown values would be. If they weren't then anybody could
look through your posts via meta with a simple
http://example.com/?meta_key=aaa&meta_value=bbb.

If you actually want that for some reason, then you'd use the
add_query_var() function to add those two names to the
public_query_vars, allowing both that and a call to wp() to use those
variables.

You probably don't want that, in which case you should be creating a
new WP_Query, or calling wp_query() to change the query.

-Otto