Wine query - find terms based off posts in $a term and then get all matching posts grouped by $b term

query_function.php

<?php

// put these functions in your themes function file or similar (plugin file maybe?)

function find_all_producers_in_this_region( $region_slug ) {
		global $post;
		/*
		* This function is written to work on any page as long as it is supplied the region slug somehow
		* 
		* If this is being displayed/used on a region's archive page, this 1st query is superflous
		* as it will be the same as the main query. You would alter this function to work within the main loop
		* then call the next function to fetch each producers wines in this region
		*
		* 
		*/

		$region_args = array(
			// not sure if you've used a custom post type, change if you have
			'post_type' => 'wine',
			'tax_query' => array(
				array(
					'taxonomy' => 'region',
					'field' => 'slug',
					'terms' => $region_slug // your term here (the region)
				)
			)
		);

		// Make the Query, fetch all posts in this region 
		$region_query = new WP_Query( $region_args );

		if ( ! $region_query->have_posts() ):

			// The query did not find any posts (wines) in this region

			// Reset just to be sure, probs not necessary
			wp_reset_postdata();

			return false;

		else:

			//Setup an empty array for us to place producer ID's into as we go through the loop
			$producers_in_this_region = array();

			while ( $region_query->have_posts() ):

				//standard 'loop' stuff
				$region_query->the_post();

				/*
				* Fetch the terms (producers) for this post (wine). 
				*
				* Even if there is only one term it will still return an array. I'm assuming only 1 
				* producer per wine, so I'm just using the first index of the array.. 
				*
				* For multiple producers per wine create a foreach() to loop through
				* $this_wines_producers, instead of using $this_wines_producers[0]
				*
				*/

				$this_wines_producers = wp_get_post_terms( $post->ID, 'producer', array( "fields" => "all" ) );

				if ( !empty( $this_wines_producers ) ) {
					
					$producers_in_this_region[] = $this_wines_producers[0]->slug;
				}

			// end of the loop
			endwhile; 

			// Remove duplicate producers
			$producers_in_this_region = array_unique( $producers_in_this_region );

			// Sort Alphabetically (case insentive)
			sort( $producers_in_this_region, SORT_STRING | SORT_FLAG_CASE );

			// Reset post data to not intefere with main/other queries
			wp_reset_postdata();

			return $producers_in_this_region;

		endif;
		
	}

	function find_and_render_wines_by_a_producer_in_this_region( $producer_slug, $region_slug ) {
		// Make sure we have what we need before we continue 
		if ( !$region_slug || !$producer_slug ):

			// You can decide what to do in the case of an error, this is just here as an example,
			// this particular error is pretty unlikely
			echo "<p class='error'> There was an error, either the producer or region were not set correctly</p>";
			return;

		else:

			//$region_slug and $producer_slug both have at least SOME value, so lets continue

			// This query will only return wines in the current region - if a producer in this region also
			// makes / has wines in other regions (not uncommon) then they will not show on this page

			// orderby value is menu_value - feel free to change, though many drag + drop post ordering
			// plugins use menu_value to set order, so you could impliment that

			$producer_args = array(
								'post_type' => 'wine',
								'orderby' 	=> 'menu_order',
								'tax_query' => array(
									'relation' => 'AND',
									array(
										'taxonomy' => 'producer',
										'field' => 'slug',
										'terms' => $producer_slug 
										),
									array(
										'taxonomy' => 'region',
										'field' => 'slug',
										'terms' => $region_slug
										)
									)
								);

			$producer_query = new WP_Query( $producer_args );

			if ( $producer_query->have_posts() ) :

				// This code runs before the wines loop, so you could use it to output markup only once per producer
				// Ideally, you could use it to provide a heading for reach producer, possibly a blurb about them etc

				// Replace this markup with whatever you like, this was just what I used for testing purposes

				$this_producer = get_term_by( 'slug', $producer_slug, 'producer' );
				echo "<h3>".$this_producer->name."</h3>";


				// This here is the loop that will put out the wines belonging to this producer in this region

				// Change the markup / output etc to whatever you want

				// You could even decouple this further so this function only does the querying and you have something 
				// else ( get_template_part() or another function ) to do the markup

				while ( $producer_query->have_posts() ) :
					$producer_query->the_post();
					
					// I just used this for testing, put your actual markup here for each wine. 
					echo "<h4>".get_the_title()."</h4>";
					echo the_content();

				endwhile; 
			endif;

			wp_reset_postdata();

		endif;
	}

	function wines_in_this_region_grouped_by_producer( $region_slug ){
		if ( ! $region_slug ):
			echo "<p class='error'> There was an error, no region is specified</p>";
			return;
		else:

			$producers = find_all_producers_in_this_region( $region_slug );

			if ( ! $producers ):
				// Producers in region query returned no results
				echo "<p class='error'> There was an error, no producers were found in this region</p>";
				echo "<pre>".print_r( $producers )."</pre>";
				return;
			else:

				// lez do dis
				foreach ( $producers as $producer_slug ){
					find_and_render_wines_by_a_producer_in_this_region( $producer_slug, $region_slug );
				}

			endif;
		endif;

	}

template_function_example.php

<?php 
	get_header();
?>
<div id="main-content" class="main-content">

	<h1> Welcome to Query Testing Page </h1>

	<div id="primary" class="content-area">
		<div id="content" class="site-content" role="main">
			<?php

				// to use in one of your theme's template files 
	
				// region slug should be set somehow - main query vars, custom field on the page etc
	
				wines_in_this_region_grouped_by_producer( 'your_region_slug_here' );

			?>
		</div><!-- #content -->
	</div><!-- #primary -->
</div><!-- #main-content -->

<?php
get_sidebar();
get_footer();