Relationships

Kohana ORM supports four types of object relationships: belongs_to, has_many, has_many "through" and has_one. The has_many "through" relationship can be used to function like Active Record's has_many_and_belongs_to relationship type.

belongs_to

A belongs_to relation should be used when you have one model that belongs to another. For example, a Child model belongs_to a Parent or a Flag model belongs_to a Country.

This is the base belongs_to relationship:

protected $_belongs_to = array(
    '[alias name]' => array(
        'model'       => '[model name]',
        'foreign_key' => '[column]',
    ),
);

You can omit any or all of the keys/values in the array on the right, in which case defaults are used:

protected $_belongs_to = array('[alias name]' => array());

The alias name is what is used to access the related model in your code. If you had a Post model that belonged to a User model and wished to use the default values of the belongs_to configuration then your code would look like this:

protected $_belongs_to = array('user' => array());

To access the user model, you would use $post->user. Since we're using the defaults above, the alias name will be used for the model name, and the foreign key in the posts table will be the alias name followed by _id, in this case it would be user_id. (You can change the _id suffix by modifying the $foreign_key_suffix variable in the model.)

Let's say your Post database table schema doesn't have a user_id column but instead has an author_id column which is a foreign key for a record in the User table. You could use code like this:

protected $_belongs_to = array(
    'user' => array(
        'foreign_key' => 'author_id',
    ),
);

If you wanted access a post's author by using code like $post->author then you would simply need to change the alias and add the model index:

protected $_belongs_to = array(
    'author' => array(
        'model'       => 'user',
    ),
);

has_many

The standard has_many relationship will likely fall on the other side of a belongs_to relationship. In the above examples, a post belongs to a user. From the user's perspective, a user has many posts. A has_many relationship is defined below:

protected $_has_many = array(
    '[alias name]' => array(
        'model'       => '[model name]',
        'foreign_key' => '[column]',
    ),
);

Again, you can omit all keys in the right array to use the defaults:

protected $_has_many = array('[alias name]' => array());

For our user and post example, this would look like the following in the user model:

protected $_has_many = array('posts' => array());

Using the above, the posts could be access using $user->posts->find_all(). Notice the find_all() used in this example. With belongs_to and has_one relationship types, the model is already loaded with necessary data. For has_many relationships, however, you may want to limit the number of results or add additional conditions to the SQL query; you can do so prior to the find_all().

The model name used by default will be the singular name of the alias using the inflector class. In this case, posts uses post as the model name. The foreign key used by default is the owner model's name followed by _id. In this case, the foreign key will be user_id and it must exist in the posts table as before.

Let's assume now you want to access the posts using the name stories instead, and are still using the author_id key as in the belongs_to example. You would define your has_many relationship as:

protected $_has_many = array(
    'stories' => array(
        'model'       => 'post',
        'foreign_key' => 'author_id',
    ),
);

has_one

A has_one relationship is almost identical to a has_many relationship. In a has_one relationship, there can be 1 and only 1 relationship (rather than 1 or more in a has_many). If a user can only have one post or story, rather than many then the code would look like this:

protected $_has_one = array(
    'story' => array(
        'model'       => 'post',
        'foreign_key' => 'author_id',
    ),
);

has_many "through"

A has_many "through" relationship is used for many-to-many relationships. For instance, let's assume now we have an additional model, called Category. Posts may belong to more than one category, and each category may have more than one post. To link them together, an additional table is needed with columns for a post_id and a category_id (sometimes called a pivot table). We'll name the model for this Post_Category and the corresponding table categories_posts.

To define the has_many "through" relationship, the same syntax for standard has_many relationships is used with the addition of a 'through' parameter. Let's assume we're working with the Post model:

protected $_has_many = array(
    'categories' => array(
        'model'   => 'category',
        'through' => 'categories_posts',
    ),
);

In the Category model:

protected $_has_many = array(
    'posts' => array(
        'model'   => 'post',
        'through' => 'categories_posts',
    ),
);

To access the categories and posts, you simply use $post->categories->find_all() and $category->posts->find_all()

Methods are available to check for, add, and remove relationships for many-to-many relationships. Let's assume you have a $post model loaded, and a $category model loaded as well. You can check to see if the $post is related to this $category with the following call:

$post->has('categories', $category);

The first parameter is the alias name to use (in case your post model has more than one relationship to the category model) and the second is the model to check for a relationship with.

Assuming you want to add the relationship (by creating a new record in the categories_posts table), you would simply do:

$post->add('categories', $category);

To remove:

$post->remove('categories', $category);