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Custom fields are great for storing various meta data about a post. However, if you routinely need to display posts that have a certain custom field value, you should consider switching to a <a href="https://codex.wordpress.org/Custom_Taxonomies" rel="nofollow ugc">custom taxonomy</a>, which is better suited for grouping posts.
With this simple plugin, you can convert all custom fields with a certain key to terms in a certain taxonomy, while maintaining the post association.
Note: If you’re looking for the old version, see <a href="https://wordpress.org/extend/plugins/custom-query-fields/" rel="ugc">Custom Query Fields</a>
Links: <a href="http://scribu.net/wordpress/custom-field-taxonomies" rel="nofollow ugc">Plugin News</a> | <a href="http://scribu.net" rel="nofollow ugc">Author’s Site</a>

Custom Field Taxonomies

Convert custom fields to tags, categories or taxonomy terms

JS Morisset

The JSM Show Term Metadata plugin displays term (ie. categories, tags, and custom taxonomies) meta keys and unserialized values in a metabox at the bottom of the term editing page.
There are no plugin settings – simply install and activate the plugin.
<h4>Shows Yoast SEO Term Meta</h4>
Yoast SEO stores its term (ie. categories, tags, and custom taxonomies) metadata in the WordPress options table, not the term meta table.
The JSM Show Term Metadata plugin can read and display Yoast SEO’s term metadata, but it cannot be deleted (as it does not reside in the WordPress term meta table).
<h4>Available Filters for Developers</h4>
Filter the term meta shown in the metabox:
<pre><code>'jsmstm_metabox_table_metadata' ( array $metadata, $term_obj )
</code></pre>
Array of regular expressions to exclude meta keys:
<pre><code>'jsmstm_metabox_table_exclude_keys' ( array $exclude_keys, $term_obj )
</code></pre>
Capability required to show term meta:
<pre><code>'jsmstm_show_metabox_capability' ( 'manage_options', $term_obj )
</code></pre>
Show term meta for a taxonomy (defaults to true):
<pre><code>'jsmstm_show_metabox_taxonomy' ( true, $taxonomy )
</code></pre>
Capability required to delete term meta:
<pre><code>'jsmstm_delete_meta_capability' ( 'manage_options', $term_obj )
</code></pre>
Icon for the delete term meta button:
<pre><code>'jsmstm_delete_meta_icon_class' ( 'dashicons dashicons-table-row-delete' )
</code></pre>
<h4>Related Plugins</h4>
<ul>
<li><a href="https://wordpress.org/plugins/jsm-show-comment-meta/" rel="ugc">JSM Show Comment Metadata</a></li>
<li><a href="https://wordpress.org/plugins/jsm-show-order-meta/" rel="ugc">JSM Show Order Metadata for WooCommerce HPOS</a></li>
<li><a href="https://wordpress.org/plugins/jsm-show-post-meta/" rel="ugc">JSM Show Post Metadata</a></li>
<li><a href="https://wordpress.org/plugins/jsm-show-term-meta/" rel="ugc">JSM Show Term Metadata</a></li>
<li><a href="https://wordpress.org/plugins/jsm-show-user-meta/" rel="ugc">JSM Show User Metadata</a></li>
<li><a href="https://wordpress.org/plugins/jsm-show-registered-shortcodes/" rel="ugc">JSM Show Registered Shortcodes</a></li>
</ul>

JSM Show Term Metadata

Show term metadata in a metabox when editing terms - a great tool for debugging issues with term metadata.

Custom Field Taxonomies vs JSM Show Term Metadata