WP_query - Filter by tax_query and meta_query using multiple select

Question

I have a Publications Post Type, a Writers Taxonomy and a Publications Custom 'Meta Box' Value; parent_id.

In my archive-publictions-post.php template I am trying to filter the WP_Query Class, with the tax_query and meta_query Parameters.

With the code below I can get all Publications to show on page load and I can get filtered results when I select both a Term and a Parent together but do not when selected individually?

I am not sure about how the meta_query & tax_query relationship parameter should be used in this case?

<?php
  // The code from: archive-publictions-post.php

  // Set Var Filter
  $publications_filter = $_POST['publications'];

  // Set Terms Arguments
  $term_args = array(
    'orderby' => 'name',    
    'order' => 'ASC',     
    'hide_empty' => false,    
    'parent' => 0,      
  );
  // Get Publications Terms
  $terms = get_terms('publications-tax', $term_args);

  $writers_args = array( 
    'post_type' => 'writers-post', 
    'posts_per_page' => -1,
    'post_status' => 'any',
    'order' => 'DESC',
  ); 
  // Get Writers Posts
  $writers = get_posts($writers_args);

    echo '<select name="publications[]" multiple>';
      // Set var as an array
      $all_writers = array();
      foreach($writers as $writer){
        // Set all terms to our array
        $all_writers[] = $writer->ID;
        echo '<option value="'.$writer->ID.'">'.$writer->post_name.'</option>';
      }
      foreach($terms as $term){
        // Set all terms to our array
        $all_terms[] = $term->name;
        echo '<option value="'.$term->name.'">'.$term->name.'</option>';
      }          
    echo '</select>';         
    echo '<input type="submit" value="go">';
  ?>
  </form>

WP_QUERY $args:

   //Start Args

    $publications_results = isset($_POST['publications']) ? $publications_filter : $all_terms;

    $publication_values=array();
    $writers=array();
    foreach ($publications_results as $key => $value) {
      if (is_numeric($value)) {
        $writers_values[] = $value;
      }
      else{
        $publications_values[] = $value;
      }
    }
    $args = array(   
      'post_type' => array('publications-post','writers-post'),
      'order' => 'DESC',
      'post_status' => 'any',
      'orderby' => 'date',
      'tax_query' => array(array(
        'taxonomy' => 'publications-tax',
        'field' => 'slug',
        'terms' => $publications_values
      ))
    ); 

    if($_POST['publications']){
      $args['meta_query'][] = 
      array(
        'key' => 'parent_id',
        'value' => $writers_values,
        'type' => 'numeric',
      );
    }

Thanks

Answer 1

My initial code did not provide default WP_query argument parameters if either Select option was not set - the empty Parameter would filter everything out.

To the best of my ability, I applied default parameter values from the initial Post and Terms queries and now the select filter is working.

I would be pleased to know if my code can be improved upon :)

  /*
  *Get Publications Filters 
  */

  $writers_filter = array();
  $publications_filter = array();

  if (isset($_POST['publications'])) {

    $publications_filter = $_POST['publications'];
    // Separate Numeric from String - i.e. Post ID and Term Name
    foreach ($publications_filter as $key => $value) {
      if (is_numeric($value)) {
        $writers_filter[] = $value;
      }
      else{
        $terms_filter[] = $value;
      }
    }
  }      

  // If Either Select Option is Empty, 
  // Assign Final Value With All Queried Results - Avoid Empty Value 

  $final_writers = array();
  $final_terms = array();
  $final_writers = (!empty($writers_filter)) ? $writers_filter : $all_writers;
  $final_terms = (!empty($terms_filter)) ? $terms_filter : $all_terms;

  /*
  *Set WP_Query Argument Parameters
  */

  $args = array(   
    'post_type' => array('publications-post','writers-post'),
    'order' => 'DESC',
    'post_status' => 'any',
    'orderby' => 'date',
    'tax_query' => array(array(
      'taxonomy' => 'publications-tax',
      'field' => 'slug',
      'terms' => $final_terms
    ))
  ); 

  if($_POST['publications']){
    $args['meta_query'][] = 
    array(
      'key' => 'parent_id',
      'value' => $final_writers,
      'type' => 'numeric',
    );
  }