term/tag not being saved for custom post type

Question

I have been using a chunk of code as a plugin that was suggested in the answer to this question. Please check the code provided below instead of the one in the question, as I may have modified it a bit. The plugin works fine for the normal post type but has an issue when used with custom-post-type. My custom post type is called "research".

To be explicit, the code is not able to save the tag/term assigned with checkboxes, when used in the single-template of custom post type "research". However it does save the tags assigned with checkboxes, when used in the single-template of the normal-post-type. I would really appreciate the correction in the code please.

FRONTEND TAGGING PLUGIN CODE :

add_action('init', 'frontend_term_assignment');
add_action('frontend_term_assignment_form', 'frontend_term_assignment_form');


function frontend_term_assignment() {
  if (
    ! isset($_POST['term_assignment_post_id']) ||
    ! intval($_POST['term_assignment_post_id']) ||
    ! current_user_can('manage_options', $_POST['term_assignment_post_id']) ||
    ! isset( $_POST['term_assignment_nonce'] ) ||
    ! wp_verify_nonce($_POST['term_assignment_nonce'], 'frontend_term_assignment')
  ) return;
  $post = get_post( $_POST['term_assignment_post_id'] );
  $taxonomy = null;
  if ( $post->post_type == 'post' ) {
    $taxonomy = 'post_tag';
  } elseif ( $post->post_type == 'research' )  {
    $taxonomy = 'research_sections';
  }
  if ( empty($taxonomy) || ! isset($_POST[$taxonomy]) ) return;
  $terms = $_POST[$taxonomy];
  wp_set_post_terms( $post->ID, $terms, $taxonomy, false);
}

function frontend_term_assignment_form() {
  $post = get_queried_object();
  if ( empty($post) || ! isset($post->ID) || ! current_user_can('manage_options', $post->ID) )
  $taxonomy = null;
  if ( $post->post_type == 'post' ) {
    $taxonomy = 'post_tag';
  } elseif ( $post->post_type == 'research' )  {
    $taxonomy = 'research_sections';
  }
  if ( empty($taxonomy) ) return;
  $tax_obj = get_taxonomy($taxonomy);
  $args = array('orderby' => 'name', 'order' => 'ASC', 'hide_empty' => false );
  $terms = get_terms($taxonomy, $args);
  $post_terms = (array)wp_get_post_terms( $post->ID, $taxonomy, array("fields" => "ids") );
  if ( ! empty($terms) ) {
    echo '<form method="post" id="frontend_term_assignment_form">';
    echo '<input type="hidden" value="' . wp_create_nonce('frontend_term_assignment') . '" name="term_assignment_nonce">';
    echo '<input type="hidden" value="' . $post->ID . '" name="term_assignment_post_id">';
    echo '<div class="' . $taxonomy . '_term_list"><ul>';
    foreach ($terms as $term ) {
      $checked = in_array($term->term_id, $post_terms) ? checked(1, 1, 0) : '';
      printf('<li><label><input type="checkbox" value="%s" name="%s[]"%s> ' . $term->name . '</label></li>', $term->slug, $taxonomy, $checked);
    }
    echo '</ul><input type="submit" id="tag-sub" value="' . esc_attr( sprintf(__('Save %s'), $tax_obj->label) ) . '" />';
    echo '</div></form>';
  }
}

ADDED NOTES : Below given is a the code that is used in the single.php templates which displays the form. Tags if assigned from the wp-backend works fine and is saved fine, the trouble is just saving tags in the frontend.

<?php do_action('frontend_term_assignment_form'); ?> 

Answer 1

wp_set_post_terms only works with the post post type, use wp_set_object_terms for all post types.