Getting posts by taxonomy

Question

I want to get a set of posts from a CPT based on a taxonomy term being present. I thought I'd use WP's get_posts() function:

$results = get_posts(array $options);

This function works fine when I specify the CPT i'm interested in:

$results = get_posts( array('post_type' => 'my_cpt_name') );

But then I tried this with my UOM custom post type and tried to filter on the metric system which is a taxonomy called "uom_system":

$results = get_posts( array('post_type' => 'uoms', 'uom_system' => 'metric-system' );

Sadly, the taxonomy section gets ignored and I get the full list of posts in the UOM posts table. What am I doing wrong?

Answer 1

I usually use the tax_query parameter:

$args = array(
    'post_type'      => 'my_cpt_name',
    'post_status'    => 'publish',
    'posts_per_page' => -1,
    'tax_query'      => array( array(
            'taxonomy' => 'uom_system',
            'field'    => 'slug',
            'terms'    => 'metric-system'
            ) )
    );

$results = get_posts( $args );

See http://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters

Answer 2

I solved my own problem by realising that the array I was passing into get_posts had some name-value pairs that were not intended for that function (aka, i was using it broadly to represent "options" to which only some of these options were for the get_posts call). It appears that rather than ignoring values it doesn't understand it stops processing further name-value pairs. Fair enough. I removed the junk name-value pairs and it works fine now.

I did like Simon's contribution around 'tax_query' ... I hadn't known of that option and I can see that this might be better syntax in certain circumstances.