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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

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Simply select a block in the WordPress editor, toggle on Dynamic Content, and select the custom field (aka post meta key) that contains the value you’d like to display. If that value changes in your database, it will automatically reflect the change on the frontend of your site.
The following blocks are currently supported: 
* Paragraph 
* Heading 
* Verse

Dynamic Block Content

Enable WordPress blocks to display dynamic data that auto-updates on the frontend of your site.

Custom Field Taxonomies vs Dynamic Block Content