filtering custom post types via meta data drop down

Question

I'm trying to add drop down filters (that are populated by terms that are in use) to a custom search page for "opportunities" or listings page. Currently I am doing this with taxonomies but doing this with meta data, meta_key, meta_query is new to me.

I've searched around and most questions are not really dealing with pre-determined filters like this.

I just found out about http://wordpress.org/extend/plugins/facetious and http://wordpress.org/extend/plugins/advance-wp-query-search-filter from this post: https://wordpress.stackexchange.com/questions/99119/advanced-wordpress-search-with-drop-down-menus but a plugin may be overkill since I've gotten the other parts of this working.

The general idea is grab all the values of "op_duration_meta" (e.g. "one-time", "short-term", "long-term") and turn it into a drop down that will sort the list or in combination with the other drop downs.

I was starting to think I was going to have to resort to appending stuff to the URL or something manually.

Thanks for any help in advance.

<div class="search">

    <form method="get" class="search-form" action="<?php echo home_url(); ?>">
    <div>
        <?php $query_types = get_query_var('post_type'); ?>

        <?php $args15 = array(
            'type'                     => 'opportunities',
            'child_of'                 => 0,
            'parent'                   => '',
            'orderby'                  => 'name',
            'order'                    => 'ASC',
            'hide_empty'               => 1,
            'hierarchical'             => 1,
            'exclude'                  => '',
            'include'                  => '',
            'number'                   => '',
            'taxonomy'                 => 'neighborhoods',
            'pad_counts'               => false );?>

        <?php $args16 = array(
            'type'                     => 'opportunities',
            'child_of'                 => 0,
            'parent'                   => '',
            'orderby'                  => 'name',
            'order'                    => 'ASC',
            'hide_empty'               => 1,
            'hierarchical'             => 1,
            'exclude'                  => '',
            'include'                  => '',
            'number'                   => '',
            'taxonomy'                 => 'post_tags',
            'pad_counts'               => false );?>


        <?php $args17 = array(
            'type'                     => 'opportunities',
            'child_of'                 => 0,
            'parent'                   => '',
            'orderby'                  => 'name',
            'order'                    => 'ASC',
            'hide_empty'               => 1,
            'hierarchical'             => 1,
            'exclude'                  => '',
            'include'                  => '',
            'number'                   => '',
            'taxonomy'                 => 'member_agencies',
            'pad_counts'               => false );?>

        <?php $args18 = array(
            'type'                     => 'opportunities',
            'child_of'                 => 0,
            'parent'                   => '',
            'orderby'                  => 'name',
            'order'                    => 'ASC',
            'hide_empty'               => 1,
            'hierarchical'             => 1,
            'exclude'                  => '',
            'include'                  => '',
            'number'                   => '',
            'taxonomy'                 => 'category',
            'pad_counts'               => false );?>

        <?php $args19 = array(
            'meta_key'                 =>'op_duration_meta',
            'post_status'              =>'publish',
            'post_type'                =>'opportunities',
            'orderby'                  =>'date',
            'order'                    =>'DESC' );
            ?>

        <div style="min-height:50px;display:block;"><label for="neighborhoods">1. Choose Neighborhood:</label><br/>
            <select name="neighborhoods">
             <option value=""><?php echo esc_attr(__('Select Neighborhood...')); ?></option>

        <?php $taxx1 = get_categories($args15);
        foreach ($taxx1 as $category) {
                $option = '<option value="'.$category->category_nicename.'">';
                $option .= $category->cat_name;
                $option .= '</option>';
                echo $option;
              }
             ?>
        </select>
        </div>
        <div style="min-height:50px;display:block;">
        <label for="member_agencies">2. OR Choose Member Agency:</label><br/>
        <select name="member_agencies">
         <option value=""><?php echo esc_attr(__('Select Member Agency...')); ?></option>
         <?php $taxx2 = get_categories($args17); foreach ($taxx2 as $category) {
                $option = '<option value="'.$category->category_nicename.'">';
                $option .= $category->cat_name;
                $option .= '</option>';
                echo $option;
              }
             ?>
        </select>
        </div>
        <div style="min-height:50px;display:block;">
        <label for="category">3. Choose Category:</label><br/>
        <select name="category">
         <option value=""><?php echo esc_attr(__('Select Category...')); ?></option>
         <?php $taxx3 = get_categories($args18); foreach ($taxx3 as $category) {
                $option = '<option value="'.$category->category_nicename.'">';
                $option .= $category->cat_name;
                $option .= '</option>';
                echo $option;
              }
             ?>
        </select>
        </div>

        <div style="min-height:50px;display:block;">
        <label for="duration">3. Choose Duration:</label><br/>
        <select name="duration">
         <option value=""><?php echo esc_attr(__('Select Duration...')); ?></option>
         <?php

              $key_1_value = get_post_meta($args19);

              foreach ($key_1_value as $category) {
                // check if the custom field has a value
                $option = '<option value="Opportunity Duration">';
                $option .= $category;
                $option .= '</option>';
                echo $option;
            }
             ?>
        </select>
        </div>

        <div><input type="hidden" name="post_type" value="opportunities" checked="checked"/>

        <input style="height:24px;display:inline-block" class="search-submit button" name="submit" type="submit" value="<?php esc_attr_e( 'Sort This List', 'prototype' ); ?>" /></div>
    </div>
    </form><!-- .search-form -->

</div><!-- .search -->

Answer 1

Because you want the results of the filtering process be the main object of your page, you may need to alter the main query using pre_get_posts action hook. It is really better that leave the main query run, load your theme and then say to WordPress: go back and run another query as suggested in other answers.

//Functions for filters
add_action( 'pre_get_posts', 'my_pre_get_post' );
function my_pre_get_post($query){

     //limit to main query, frontend and archive pages. You need another set of checkings, like check if the request is for your custom post type
     if($query->is_main_query() && !is_admin() && $query->is_archive ) {

          $meta_query = array();
          $meta_input = isset($_POST['meta_input']) ? $_POST['meta_input'] : '';

          if(!empty($meta_input)){
              $meta_query[] = array(
                  'key'  => 'your_meta_key',
                  //sanitize $_POST['meta_input'] according with your data type
                  'value'    => $_POST['meta_input'],
              );
          } 
          $query->set('meta_query',$meta_query);
    }

}

See more information about the meta_query.