custom search form, posts_per_page value being ignored

Question

I'm trying to build a custom search form and I'd like to override the site's default posts per page limit but the hidden field I'm using doesn't seem to want to work at all:

I have:

<input type="hidden" value="-1" name="posts_per_page" />

..but this seems to be ignored when I'm inspecting $wp_query on the search results page (the search I'm doing matches 29 posts, but a count on $wp_query->posts comes in at 10. I've done multiple tests using different values to no avail.

The full custom search form looks like this:

<form role="search" method="get" id="search" action="/" class="col-xs-12">
    <input type="text" value="" name="s" id="s" />
    <input type="hidden" value="1" name="sentence" />
    <input type="hidden" value="-1" name="posts_per_page" />
    <input type="hidden" value="any" name="post_type" />
    <input type="submit" id="searchsubmit" value="Search" />
</form>

Is there something pretty basic I'm overlooking?

I had a bunch of pre_get_posts actions set up for tweaking stuff in the admin screens, but I've disabled all of these to see if they were interfering but again, no joy.

Answer 1

You can set the posts_per_page with pre_get_posts action like this:

add_action( 'pre_get_posts', 'my_set_posts_per_page' );

function my_set_posts_per_page( $query ){
  if ( is_admin() ){
    return;
  }

  //Check if this is a search
  if ( isset( $query->query['s'] ) ) {
    $query->set( 'posts_per_page', -1 );
  }
}