Drupal 7 Module Development

Chapter 9
Writing a complete node access module, on the other hand, is a more complex task,
and one that we will examine in detail in the rest of this chapter. The advantages to
writing a node access module are:
• Node access modules can work in conjunction with other modules to provide
flexible access control rules.
• Node access rules set by one module may be modified by any other module,
whereas hook_node_access() implementations cannot.
• Node access modules typically address common usage scenarios and are
therefore re-usable by more people. While this is not a technical advantage,
it is a great reason to contribute code to the Drupal project.
The {node_access} table and its role
The primary difference between access control modules and node access modules
is the use of the {node_access} table. When a node access module is installed,
database queries for the {node} table can be JOINed to the {node_access}
table in order to delimit the list of returned values.
By default, the {node_access} table contains a single record which allows all
content to be viewed by all users (except those who cannot access content,
as we saw when looking at the node_access() function).
When a node access module is enabled, however, this default record is removed,
and node queries will be modified appropriately. Let's look again at a sample
query-builder function that returns a simple list of published nodes:
$select = db_select('node', 'n')
->fields('n', array('nid'))
->condition('promote', 1)
->condition('status', 1)
->orderBy('sticky', 'DESC')
->orderBy('created', 'DESC')
->extend('PagerDefault')
->limit(10)
->addTag('node_access');
$nids = $select->execute()->fetchCol();
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