Drupal 7 Module Development

Working with Content
Note here as well that we're explicitly declaring the aid field of the revision table
to be a foreign key to the aid field of the artwork table. Although Drupal does not
leverage foreign key information itself, other modules may do so. By convention,
tables in Drupal should be singular nouns.
With these two tables defined in artwork_schema(), Drupal will automatically
create the corresponding tables for us in the database when the module is first
enabled. If our module is uninstalled completely, it will also take care of removing
them for us.
Declaring our entity
There are two parts to telling Drupal about our new entity. The first is another
definition hook called hook_entity_info(). This hook tells Drupal about the
entity or entities we're providing, and also provides the Field UI system with the
information it needs to allow us to attach fields to entities—more on that later. The
second part is a "controller class", which is a PHP class that will be responsible for
loading and, in our case, creating, saving, and deleting our artwork.
Drupal includes a controller class called DrupalDefaultEntityController that
handles the most common case, which we will be emulating. It is extremely basic,
however, and only handles loading of objects. Fortunately it is very easy to subclass
the default controller and add our own functionality so that is precisely what we
will do.
A controller is a loader object for an entity. All entity types must have
a controller, but many can use the default. Different controllers may
require additional keys on an entity definition.
The entity declaration
First let's tell Drupal about our entity type using hook_entity_info():
/**
* Implements hook_entity_info().
*/
function artwork_entity_info() {
$return['artwork'] = array(
'label' => t('Artwork'),
'controller class' => 'ArtworkController',
'base table' => 'artwork',
'revision table' => 'artwork_revision',
'uri callback' => 'artwork_uri',
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