7. Tracking changes to entities

Drupal provides a mechanism to track changes of entities by storing a new revision of an entity each time it is saved.

7.1. Make events revisionable

Add the following use statements to src/Entity/Event.php:
```
use Drupal\Core\Entity\RevisionLogEntityTrait;
```
Add the following to the entity_keys of the Event class attributes:
```
'revision' => 'revision_id',
```

Add the following to the attributes of the Event class:

revision_table: 'event_revision',
show_revision_ui: TRUE,
revision_metadata_keys: [
  'revision_user' => 'revision_author',
  'revision_created' => 'revision_created',
  'revision_log_message' => 'revision_log_message',
],

Add , RevisionLogEntityTrait to the use part inside of the Event class
Add the following before the return statement of the baseFieldDefinitions() method of the Event class:
```
 $fields += static::revisionLogBaseFieldDefinitions($entity_type);
```
Run drush entity:updates
Verify that the event_revision table was created
Verify that the Revision information vertical tab appears on the event edit form

7.2. Add a user interface for managing revisions

Add the following use statements to src/Entity/Event.php:
```
use Drupal\entity\Routing\RevisionRouteProvider;
```
Add the following to the route_provider entry of the handlers section of the attributes of the Event class:
```
'revision' => RevisionRouteProvider::class,
```

Add the following to the links section of the annotation of the Event class:

'version-history' => '/event/{event}/revisions',
'revision' => '/event/{event}/revisions/{event_revision}',
'revision-revert-form' => '/event/{event}/revisions/{event_revision}/revert',