<?php
/**
 * Add meta fields support in rest API for post type `Post`
 *
 * This function will allow custom parameters within API request URL. Add post meta support for post type `Post`.
 * 
 * > How to use?
 * http://mysite.com/wp-json/wp/v2/posts?meta_key=<my_meta_key>&meta_value=<my_meta_value>
 * 
 * > E.g. Get posts which post meta `already-visited` value is `true`.
 *
 * Request like: http://mysite.com/wp-json/wp/v2/post?meta_key=already-visited&meta_value=true
 *
 * @since 1.0.0
 * 
 * @link    https://codex.wordpress.org/Class_Reference/WP_Query
 * 
 * @see     Wp-includes/Rest-api/Endpoints/Class-wp-rest-posts-controller.php
 * 
 * @param   array   $args       Contains by default pre written params.
 * @param   array   $request    Contains params values passed through URL request.
 * @return  array   $args       New array with added custom params and its values.
 */
if( ! function_exists( 'post_meta_request_params' ) ) :
	function post_meta_request_params( $args, $request )
	{
		$args += array(
			'meta_key'   => $request['meta_key'],
			'meta_value' => $request['meta_value'],
			'meta_query' => $request['meta_query'],
		);

	    return $args;
	}
	add_filter( 'rest_post_query', 'post_meta_request_params', 99, 2 );
	// add_filter( 'rest_page_query', 'post_meta_request_params', 99, 2 ); // Add support for `page`
	// add_filter( 'rest_my-custom-post_query', 'post_meta_request_params', 99, 2 ); // Add support for `my-custom-post`
endif;