Associations

Paris provides a simple API for one-to-one, one-to-many and many-to-many relationships (associations) between models. It takes a different approach to many other ORMs, which use associative arrays to add configuration metadata about relationships to model classes. These arrays can often be deeply nested and complex, and are therefore quite error-prone.

Instead, Paris treats the act of querying across a relationship as a behaviour, and supplies a family of helper methods to help generate such queries. These helper methods should be called from within methods on your model classes which are named to describe the relationship. These methods return ORM instances (rather than actual Model instances) and so, if necessary, the relationship query can be modified and added to before it is run.

Summary

The following list summarises the associations provided by Paris, and explains which helper method supports each type of association:

One-to-one

Use has_one in the base, and belongs_to in the associated model.

One-to-many

Use has_many in the base, and belongs_to in the associated model.

Many-to-many

Use has_many_through in both the base and associated models.

Below, each association helper method is discussed in detail.

Has-one

One-to-one relationships are implemented using the has_one method. For example, say we have a User model. Each user has a single Profile, and so the user table should be associated with the profile table. To be able to find the profile for a particular user, we should add a method called profile to the User class (note that the method name here is arbitrary, but should describe the relationship). This method calls the protected has_one method provided by Paris, passing in the class name of the related object. The profile method should return an ORM instance ready for (optional) further filtering.

class Profile extends Model {
}

class User extends Model {
    public function profile() {
        return $this->has_one('Profile');
    }
}

The API for this method works as follows:

// Select a particular user from the database
$user = Model::factory('User')->find_one($user_id);

// Find the profile associated with the user
$profile = $user->profile()->find_one();

By default, Paris assumes that the foreign key column on the related table has the same name as the current (base) table, with _id appended. In the example above, Paris will look for a foreign key column called user_id on the table used by the Profile class. To override this behaviour, add a second argument to your has_one call, passing the name of the column to use.

Has many

One-to-many relationships are implemented using the has_many method. For example, say we have a User model. Each user has several Post objects. The user table should be associated with the post table. To be able to find the posts for a particular user, we should add a method called posts to the User class (note that the method name here is arbitrary, but should describe the relationship). This method calls the protected has_many method provided by Paris, passing in the class name of the related objects. Pass the model class name literally, not a pluralised version. The posts method should return an ORM instance ready for (optional) further filtering.

class Post extends Model {
}

class User extends Model {
    public function posts() {
        return $this->has_many('Post'); // Note we use the model name literally - not a pluralised version
    }
}

The API for this method works as follows:

// Select a particular user from the database
$user = Model::factory('User')->find_one($user_id);

// Find the posts associated with the user
$posts = $user->posts()->find_many();

By default, Paris assumes that the foreign key column on the related table has the same name as the current (base) table, with _id appended. In the example above, Paris will look for a foreign key column called user_id on the table used by the Post class. To override this behaviour, add a second argument to your has_many call, passing the name of the column to use.

Belongs to

The ‘other side’ of has_one and has_many is belongs_to. This method call takes identical parameters as these methods, but assumes the foreign key is on the current (base) table, not the related table.

class Profile extends Model {
    public function user() {
        return $this->belongs_to('User');
    }
}

class User extends Model {
}

The API for this method works as follows:

// Select a particular profile from the database
$profile = Model::factory('Profile')->find_one($profile_id);

// Find the user associated with the profile
$user = $profile->user()->find_one();

Again, Paris makes an assumption that the foreign key on the current (base) table has the same name as the related table with _id appended. In the example above, Paris will look for a column named user_id. To override this behaviour, pass a second argument to the belongs_to method, specifying the name of the column on the current (base) table to use.

Has many through

Many-to-many relationships are implemented using the has_many_through method. This method has only one required argument: the name of the related model. Supplying further arguments allows us to override default behaviour of the method.

For example, say we have a Book model. Each Book may have several Author objects, and each Author may have written several Books. To be able to find the authors for a particular book, we should first create an intermediary model. The name for this model should be constructed by concatenating the names of the two related classes, in alphabetical order. In this case, our classes are called Author and Book, so the intermediate model should be called AuthorBook.

We should then add a method called authors to the Book class (note that the method name here is arbitrary, but should describe the relationship). This method calls the protected has_many_through method provided by Paris, passing in the class name of the related objects. Pass the model class name literally, not a pluralised version. The authors method should return an ORM instance ready for (optional) further filtering.

class Author extends Model {
    public function books() {
        return $this->has_many_through('Book');
    }
}

class Book extends Model {
    public function authors() {
        return $this->has_many_through('Author');
    }
}

class AuthorBook extends Model {
}

The API for this method works as follows:

// Select a particular book from the database
$book = Model::factory('Book')->find_one($book_id);

// Find the authors associated with the book
$authors = $book->authors()->find_many();

// Get the first author
$first_author = $authors[0];

// Find all the books written by this author
$first_author_books = $first_author->books()->find_many();

Overriding defaults

The has_many_through method takes up to four arguments, which allow us to progressively override default assumptions made by the method.

First argument: associated model name - this is mandatory and should be the name of the model we wish to select across the association.

Second argument: intermediate model name - this is optional and defaults to the names of the two associated models, sorted alphabetically and concatenated.

Third argument: custom key to base table on intermediate table - this is optional, and defaults to the name of the base table with _id appended.

Fourth argument: custom key to associated table on intermediate table - this is optional, and defaults to the name of the associated table with _id appended.