The information below is copied from the API documentation on has_many associations
http://api.akelos.org/ActiveRecord/Associations/AkHasMany.html

$has_many adds the following methods for retrieval and query of collections of associated objects.

'collection' is replaced with the singular form of current association, so var $has_many = 'clients' would hold an array of objects on $this→clients and a collection handling interface instance on $this→client (singular form)

• collection→load($force_reload = false) - returns an array of all the associated objects. An empty array is returned if none are found.

• collection→add($object, ?) - adds one or more objects to the collection by setting their foreign keys to the collection's primary key. (collection→push and $collection→concat are aliases to this method).

• collection→delete($object, ?) - removes one or more objects from the collection by setting their foreign keys to NULL. This will also destroy the objects if they’re declared as belongs_to and dependent on this model.

• collection→set($objects) - replaces the collections content by deleting and adding objects as appropriate.

• collection→setByIds($ids) - replace the collection by the objects identified by the primary keys in ids

• collection→clear() - removes every object from the collection. This destroys the associated objects if they are 'dependent', deletes them directly from the database if they are 'dependent' ⇒ 'delete_all', and sets their foreign keys to NULL otherwise.

• collection→isEmpty() - returns true if there are no associated objects.

• collection→getSize() - returns the number of associated objects.

• collection→find() - finds an associated object according to the same rules as ActiveRecord→find.

• collection→count() - returns the number of elements associated. (collection→size() is an alias to this method)

• collection→build($attributes = array()) - returns a new object of the collection type that has been instantiated with attributes and linked to this object through a foreign key but has not yet been saved. *Note:* This only works if an associated object already exists, not if it?s null

• collection→create($attributes = array()) - returns a new object of the collection type that has been instantiated with attributes and linked to this object through a foreign key and that has already been saved (if it passed the validation). *Note:* This only works if an associated object already exists, not if it?s null

A Firm class declares has_many clients, which will add:

$Firm->client->load() (similar to $Clients->find('all', array('conditions' => 'firm_id = '.$id)) )
$Firm->client->add()
$Firm->client->delete()
$Firm->client->assign()
$Firm->client->assignByIds()
$Firm->client->clear()
$Firm->client->isEmpty() (similar to count($Firm->clients) == 0)
$Firm->client->getSize() (similar to Client.count "firm_id = #{id}")
$Firm->client->find() (similar to $Client->find($id, array('conditions' => 'firm_id = '.$id)) )
$Firm->client->build() (similar to new Client(array('firm_id' => $id)) )
$Firm->client->create() (similar to $c = new Client(array('firm_id' => $id)); $c->save(); return $c )

The declaration can also include an options array to specialize the behavior of the association.

• 'class_name' - specify the class name of the association. Use it only if that name can't be inferred from the association name. So “$has_many = 'products'” will by default be linked to the Product class, but if the real class name is SpecialProduct, you?ll have to specify it with this option.

• 'conditions' - specify the conditions that the associated objects must meet in order to be included as a “WHERE” sql fragment, such as “price > 5 AND name LIKE ?B%?”.

• 'order' - specify the order in which the associated objects are returned as a “ORDER BY” sql fragment, such as “last_name, first_name DESC”

• 'group' - specify the attribute by which the associated objects are returned as a “GROUP BY” sql fragment, such as “category”

• 'foreign_key' - specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a has_many association will use “person_id” as the default foreign_key.

• 'dependent' - if set to 'destroy' all the associated objects are destroyed alongside this object by calling their destroy method. If set to 'delete_all' all associated objects are deleted without calling their destroy method. If set to 'nullify' all associated objects? foreign keys are set to NULL without calling their save callbacks.

• 'finder_sql' - specify a complete SQL statement to fetch the association. This is a good way to go for complex associations that depend on multiple tables. Note: When this option is used, findInCollection is not added.

• 'counter_sql' - specify a complete SQL statement to fetch the size of the association. If +'finder_sql'+ is specified but +'counter_sql'+, +'counter_sql'+ will be generated by replacing SELECT ? FROM with SELECT COUNT(*) FROM.

• 'include' - specify second-order associations that should be eager loaded when the collection is loaded.

• 'group' An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.

• 'limit' An integer determining the limit on the number of rows that should be returned.

• 'offset' An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.

• 'select' By default, this is * as in SELECT * FROM, but can be changed if you for example want to do a join, but not include the joined columns.

$has_many = array(
            'comments'  => array('order' => 'posted_on', 'include' => 'author', 'dependent' => 'nullify'),
            'people'    => array('conditions' => 'deleted = 0', 'order' => 'name'),
            'tracks'    => array('order' => 'position', 'dependent' => 'destroy'),
            'members'   => array('class_name' => 'Person', 'conditions' => 'role = "member"'));