Drupal views: Merge contextual filter with exposed filters in query.

drupal.views.filter.merge.php

/**
 * @file: MY_MODULE.views.inc
 *
 * Implementation of hook_views_query_alter().
 * 1. Resolves conflict between contextual filter and regular filter (topic id, sub topic).
 * Both refer to same database field, so merge them in one 'where' clause instead of two that Views comes up with.
 * 2. By default, show all results from a topic and its sub-topics. 
 * [If a document is tagged with a sub-topic but not its parent topic, it was not shown in the default result.]
 * @param view $view
 * @param views_plugin_query_default $query 
 */
function MY_MODULE_views_query_alter(&$view, &$query) {
  $topics_tid_a = array();
  if ($view->name == 'MY_VIEW' && $view->current_display=='default') {
    if(count($view->exposed_raw_input) && count($view->exposed_raw_input['field_topics_tid'])) {
      $topics_tid_a = array_merge($topics_tid_a, $view->exposed_raw_input['field_topics_tid']);
      
      foreach($query->where as $k => $condition) {
        if($k==0) continue; // first group is for contexual filters
        foreach($condition['conditions'] as $i => $c) {
          if(strpos($c['field'],'field_data_field_topics.field_topics_tid') !== false) {
            unset($query->where[$k]['conditions'][$i]);
          }
        }
      }
    } else {
      $topics_tid_a = $view->args;
      $options = _get_available_filter_options();
      if(count($options['subtopic'])) {
        $topics_tid_a = array_merge($topics_tid_a, array_keys($options['subtopic']));
      }
    }
    
    $query->where[0]['conditions'][0]['field'] = 'field_data_field_topics.field_topics_tid';
    $query->where[0]['conditions'][0]['operator'] = 'in';
    $query->where[0]['conditions'][0]['value'] = $topics_tid_a;
  }
}


/**
 * @file: MY_MODULE.module
 *
 * Implemented hook_views_api().
 * @return type
 */
function MY_MODULE_views_api() {
  return array(
    'api' => 3,
  );
}

This was super helpful. Thank you for sharing.