Comment settings are now a field. Comments allowed on any entity type.

In Drupal 7, comment settings were per node type. The settings were managed from the 'edit node type' form and saved as variables.

In Drupal 8, comment settings are a field. This means you can add commenting to any entity type by adding a comments field. This also means you can have multiple comment threads on a single entity type so you could potentially have a 'for' and an 'against' comment field. Or a 'public' and 'admin' comments.

User interface changes

• Comment field ui now has its own distinct place, on the same level as content type settings.
• Comment settings are no longer managed on the content type settings page, instead these are field instance settings and are managed by editing the comment field on an entity

Field settings

Admin settings

Upgrade path

The upgrade path is written so that all node types that have comments enabled by default gets a new comment field created for it (even if no nodes of that type have any comments). Comment settings are migrated over to that field's instance settings. One comment-field is created for each node type because in Drupal 7 it was possible to have different fields on each form so this preserves that functionality.

Fields on comment-forms

Each field added must nominate a comment type (bundle) as this governs the fields present on the comment-form. In the default install a new comment field called 'comment' is added to the article and page node type. You can utilise the 'reuse existing field' form on new node-types to add this field to node types you create. Fields attached to the comment-form are per comment-type. So if you wish to have the same comment-form on article and page node-type, you would use the same comment-type on both. If you want to have a 'link' field (for example) on the article node comment form, but not on the page node comment form, then add a new comment-type and then choose this when adding the new 'comments' field to the page content-type. Each comment-type allows a separate set of fields on the comment-form.
Managing fields on comment-forms is done from the new admin UI at admin/structure/comments (see screenshot above).

API Changes

There are significant API changes in this change - including the introduction of a CommentManager service. Whilst procedural wrappers remain, we will be working to continue to move many of these to the manager service to allow proper DI (dependency injection). Procedural wrappers will be retained and marked as deprecated. Many of these procedural wrappers have changed from 8.0-ALPHA3 to accommodate the new functionality. Key changes are as follows:

Contstants changed

COMMENT_NODE_OPEN => COMMENT_OPEN
COMMENT_NODE_CLOSED => COMMENT_CLOSED
COMMENT_NODE_HIDDEN => COMMENT_HIDDEN

To determine if an entity has a comment field

Use comment manager service.

// Check if there are any comment fields on nodes, returns array of field info 
// consistent with field_info_fields() filtered to only comment fields.
$field_names =\Drupal::service('comment.manager')->getFields('node');
// Do something with a particular field.
if (isset($field_names['my_comment_field'])) {
  $comment_count = $node->my_comment_field->comment_count;
}

Creating a comment field using code

The simplest way is to use the manager.

// Add a field using default name of comment and default value of hidden.
\Drupal::service('comment.manager')->addDefaultField('node', 'page');
// Add a second comment field called admin_comments, defaulting to open.
\Drupal::service('comment.manager')->addDefaultField('node', 'page', 'admin_comments', COMMENT_OPEN);

Comment configuration

All variables of form comment_XXX_%type are removed, use instance settings instead.

D7

$mode = variable_get('comment_default_mode_' . $node->type, COMMENT_MODE_THREADED);

D8

use \Drupal\field\Field.
$instance = Field::fieldInfo()->getInstance('node', $node->type, 'comment');
$mode = $instance->getFieldSetting('default_mode');

Forum

Forum creates its own comment field called comment_forum.

D7

  if ($node->comment == COMMENT_NODE_OPEN) {
    // Do something.
  }

D8

  if ($node->comment_forum->status == COMMENT_OPEN) {
    // Do something.
  }

Comment statistics

Because we can have multiple comment fields on an entity/bundle - the comment_count etc (anything that was in node_comment_statistics table) have been moved from top level node object properties to properties of each field. Note that node_comment_statistics table is now comment_entity_statistics for consistency.

D7

$comment_count = $node->comment_count;

D8

$field_name = 'comment';
$comment_count = $node->$field_name->comment_count;