Drupal 7 Module Development

Chapter 7
The most important magic callback for a field type is the schema callback, its
definition can be seen in the following example:
function dimfield_field_schema($field) {
if ($field['type'] == 'dimensions') {
$schema['columns']['height'] = array(
'type' => 'int',
'not null' => FALSE,
);
$schema['columns']['width'] = array(
'type' => 'int',
'not null' => FALSE,
);
$schema['indexes'] = array(
'height' => array('height'),
'width' => array('width'),
);
if ($field['settings']['num_dimensions'] == 3) {
$schema['columns']['depth'] = array(
'type' => 'int',
'not null' => FALSE,
);
$schema['indexes']['depth'] = array('depth');
}
$schema['columns']['units'] = array(
'type' => 'varchar',
'length' => 10,
'not null' => FALSE,
);
}
}
return $schema;
As we would expect from a name like hook_field_schema(), its return value is a
Drupal schema array. Although fields will not always be saved in an SQL database,
they usually are, and it's a convenient syntax to reuse. Note that in this case, we
define two database columns, for height and width, and possibly a third for depth
if our field is configured to have three dimensions. (We will skip over supporting
four or five dimensions for now as it is an edge case.) The difference in the data
structure is the reason the number of dimensions are a field setting rather than a
field instance setting.
[ 187 ]