Paginate results filtered by condition on associated model (HABTM) using Containable

Question

I need to paginate list of Products belonging to specific Category (HABTM association).

In my Product model I have

var $actsAs = array('Containable');
var $hasAndBelongsToMany = array(
    'Category' => array(
        'joinTable' => 'products_categories'
    )
);

And in ProductsController

$this->paginate = array(
    'limit' => 20,
    'order' => array('Product.name' => 'ASC'),
    'contain' => array(
        'Category' => array(
            'conditions' => array(
                'Category.id' => 3
            )
        )
    )
);
$this->set('products', $this->paginate());

However, resulting SQL looks like this:

SELECT COUNT(*) AS `count` 
FROM `products` AS `Product` 
WHERE 1 = 1;

SELECT `Product`.`*` 
FROM `products` AS `Product` 
WHERE 1 = 1 
ORDER BY `Product`.`name` ASC 
LIMIT 20;

SELECT `Category`.`*`, `ProductsCategory`.`category_id`, `ProductsCategory`.`product_id` 
FROM `categories` AS `Category` 
JOIN `products_categories` AS `ProductsCategory` ON (`ProductsCategory`.`product_id` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20) AND `ProductsCategory`.`category_id` = `Category`.`id`)
WHERE `Category`.`id` = 3

(I.e. it selects 20 Products and then queries their Categories)

while I'd need

SELECT COUNT(*) AS `count` 
FROM `products` AS `Product` 
JOIN `products_categories` AS `ProductsCategory` ON `ProductsCategory`.`product_id` = `Product`.`id`
JOIN `categories` AS `Category` ON `Category`.`id` = `ProductsCategory`.`category_id`
WHERE `Category`.`id` = 3;

SELECT `Product`.*, `Category`.*
FROM `products` AS `Product` 
JOIN `products_categories` AS `ProductsCategory` ON `ProductsCategory`.`product_id` = `Product`.`id`
JOIN `categories` AS `Category` ON `Category`.`id` = `ProductsCategory`.`category_id`
WHERE `Category`.`id` = 3
ORDER BY `Product`.`name` ASC 
LIMIT 20;

(I.e. select top 20 Products which belong to Category with id = 3)

Note: Possible solution without Containable would be (as Dave suggested) using joins. This post offers a very handy helper to build $this->paginate['joins'] to paginate over HABTM association.

Note: Still looking for more elegant solution using Containable than fake hasOne binding.

Answer 1

Finally I found a way to do what I want, so posting it as an answer:

To force JOIN (and be able to filter via condition on associated model) in Containable - you've got to use fake hasOne association.

In my case, code in ProductsController should be:

$this->Product->bindModel(array('hasOne' => array('ProductsCategory')), false);

$this->paginate = array(
    'limit' => 20,
    'order' => array('Product.name' => 'ASC'),
    'conditions' => array(
        'ProductsCategory.category_id' => $category
    ),
    'contain' => 'ProductsCategory'
);

$this->set('products', $this->paginate());

Note false as a second argument to bindModel - which makes binding persistent. This is needed because paginate() issues find('count') before find('all'), which would reset temporary binding. So you might want to manually unbindModel afterwards.

Also, if your condition includes multiple IDs in HABTM associated model, you might want to add 'group' => 'Product.id' into your $this->paginate[] (as Aziz has shown in his answer) to eliminate duplicate entries (will work on MySQL only).

UPDATE: However, this approach has one serious drawback compared to joins approach (suggested by Dave): condition can apply only to intermediate model's foreign key (category_id in my case); if you want to use condition on any other field in associated model - you'd probably have to add another bindModel('hasOne'), binding intermediate model to HABTM associated model.