Can I set my WP_Query to be a Main Query?

Question

When I create a WP_Query it's automatically considered a "secondary query", but what if I replace my main query with a WP_Query? Can I set my WP_Query to be considered a "Main Query"?

The reason I say this is because I need to use pre_get_posts to do some filtering. My current logic looks like this:

if(isset($query->query_vars['post_type']) && $query->query_vars['post_type'] == 'my_post_type')

The problem with the above logic is that I have 2 queries in the same template of the same post type but only one of them is my core query, the query I want to label as "Main Query" so I can use a conditional such as $query->is_main_query() to differentiate the two and filter accordingly.

Answer 1

This is an overly complicated approach. pre_get_posts will work for any query and there are numerous ways to control when/how the callback behaves:

1. Add and remove the callback:

   function dummy_action($q) {
   
   }
   
   add_action('pre_get_posts','dummy_action');
   $q = new WP_Query(
     array(
       'post_type' => 'post'
     )
   );
   remove_action('pre_get_posts','dummy_action');
   

2. Use a self-removing callback:

   function dummy_action($q) {
     remove_action('pre_get_posts','dummy_action');
   }
   
   add_action('pre_get_posts','dummy_action');
   $q = new WP_Query(
     array(
       'post_type' => 'post'
     )
   );
   

3. Feed the query a parameter you can use as a trigger:

   function dummy_action($q) {
     if ($q->get('action_trigger')) {
       // do something
     }
   }
   
   add_action('pre_get_posts','dummy_action');
   $q = new WP_Query(
     array(
       'post_type' => 'post',
       'action_trigger' => true
     )
   );
   

You do not want to "assign" a secondary query as the main query. That will cause the same issues as caused by using query_posts(), which essentially does assign a secondary query as the main query.