WordPress: Nested field queries - An idea for a plugin to support the "field_query" argument in the WP_Query class

field_query.php

<?php
/**
 * Plugin Name: 	Field Query
 * Description: 	Support the "field_query" argument in the WP_Query class
 * Plugin URI:  	https://gist.github.com/birgire/ebf79ab6b12258a0e489
 * Plugin Author: 	Birgir Erlendsson (birgire)
 * Version: 		Proposal (2015-07-28)
 **/
 
/**
 * A plugin proposal to support the "field_query" argument in the WP_Query class
 *
 * It should support: 
 *   - fields in the wp_posts table
 *   - nested queries (like tax_query and meta_query)
 *   - tax queries (tax_query using the WP_Tax_query class )
 *   - meta queries (meta_query using the WP_Meta_query class)
 *   - date queries (date_query using the WP_Date_query class) 
 *   - comparition: =, <, >, !=, <>, <=, >=, LIKE, RLIKE, REGEXP
 */

Example 1 
----------

$args =[
  'field_query' => [
		'outer_relation' => 'AND',
		'relation'       => 'OR',
		[ 
			'field'   => 'title',
			'value'   => '%foo%',
			'compare' => 'LIKE',
		],
		[ 
			'field'   => 'content',
			'value'   => '%foo%',
			'compare' => 'LIKE',
		],
	]
];

Generated SQL:
	{SQL FROM WP_Query} AND ( post_title LIKE '%foo%' OR post_content LIKE '%foo%' )

Example 2 
----------

$args =[
  'field_query' => [
		'outer_relation' => 'OR',
		'relation'       => 'AND',
		[ 
			'field'   => 'title',
			'value'   => '%foo%',
			'compare' => 'LIKE',
		],
		[ 
			'field'   => 'ID',
			'value'   => '100',
			'compare' => '>=',
		],
	]
];

Generated SQL:
	{SQL FROM WP_Query} AND ( post_title LIKE '%foo%' AND ID >= 100 )


Example 3 (Nested)
----------

$args =[
  'field_query' => [
		'outer_relation' => 'AND',
		'relation'       => 'OR',
		[
			'relation'       => 'AND',
			[ 
				'field'   => 'title',
				'value'   => '%foo%',
				'compare' => 'LIKE',
			],
			[ 
				'field'   => 'ID',
				'value'   => '100',
				'compare' => '>=',
			],

		],
		[
			'relation'       => 'AND',
			[ 
				'field'   => 'title',
				'value'   => '%foo%',
				'compare' => 'LIKE',
			],
			[ 
				'field'   => 'content',
				'value'   => '%foo%',
				'compare' => 'LIKE',
			],
		]
	]
];

Generated SQL:
	{SQL FROM WP_Query} AND ( 
			( post_title LIKE '%foo%' AND ID >= 100 )
			  OR
			  ( post_title LIKE '%foo%' AND post_content LIKE '%foo%' )
			)

Example 4 (with Tax Query)
----------
$args =[
  'field_query' => [
		'outer_relation' => 'AND',
		'relation'       => 'OR',
		[ 
			'field'   => 'title',
			'value'   => '%foo%',
			'compare' => 'LIKE',
		],
		[ 
			'field'   => 'content',
			'value'   => '%foo%',
			'compare' => 'LIKE',
		],
		[ 
			'field'     => 'tax',
			'tax_query' => [
				[
					'taxonomy' => 'category',
					'field'    => 'slug',
					'terms'    => 'foo',			
				]
			]
		],
	]
];

Generated SQL:
	{SQL FROM WP_Query} AND ( post_title LIKE '%foo%' OR post_content LIKE '%foo%' OR {TaxQuery} )