Filter out posts according to permissions and categories

filter-out.php

<?php

add_action( 'pre_get_posts', 'mw_post_limiter' );
function mw_post_limiter( $wp_query ) {
 	// only modify for the main query - if we're not there, bail.
	if ( ! $wp_query->is_main_query() )
		return;
  
  // category_slug => permission
  $filtration_pairs = array(
    'friend' => 'view_friend_content',
    'inner-circle' => 'view_inner_content',
    'family' => 'view_family_content'
  );
  
  $not_in = array();
  
  foreach( $filtration_pairs as $cat_slug => $permission ) {
    if ( current_user_can( $permission ) )
      continue;
    $not_in[] = $cat_slug;
  }
  
  // if we made it through the above without tripping any permissions, just bail
  if ( empty( $not_in ) )
    return;

	// first grab any tax_query that might already be set. being a good citizen and stuff.
	$tax_query = (array) $wp_query->get( 'tax_query' );
	$tax_query[] = array(
		'taxonomy' => 'category',
		'field' => 'slug',
		'terms' => $not_in,
		'operator' => 'NOT IN'
	);
  // now set it
	$wp_query->set( 'tax_query', $tax_query );
}