Multiple relationships in CakePHP

Question

Suppose (for lack of better example) I have a model Person and another model Twin (meaning a pair of twins). Twins has two Person foreign keys, say first_born_id and second_born_id referencing the id fields of two different people (in Person). How do I set up the relationships in cake?

I guess Twin would have something like:

$belongsTo = array('FirstBorn' => array('className' => 'Person',
                                        'foreignKey' => 'firstborn_id'),
                   'SecondBorn' => array('className' => 'Person',
                                         'foreignKey' => 'secondborn_id'));

But how should I set up Person? I could do it like:

$hasOne = array('TwinAsFirstborn' => array('className' => 'Twin',
                                           'foreignKey' => 'firstborn_id'),
                'TwinAsSecondborn' => array('className' => 'Twin',
                                           'foreignKey' => 'secondborn_id'));

But then when I have a Person and I want to know about its twin I would need to check both relationships. I guess I am hoping there is a way to get a "Twin" relationship in Person representing any of the two ways a Person can be in a Twin relationship.

Or is there a better way to set this up?

Answer 1

I agree with the Twin example being a little confusing. Let me assume that you have a Product model (Twin in your example) and it always has 2 Component models attached to it.

components: id - name
products: id - component1_id - component2_id

I would set up Product as follows:

var $belongsTo = array(
    'Component1' => array(
        'className' => 'Component',
        'foreignKey' => 'component1_id'
    ),
    'Component2' => array(
         'className' => 'Component',
         'foreignKey' => 'component2_id'
    )
);

And Component as:

var $hasMany = array(
    'ProductWithComponentAsComponent1' => array(
        'className' => 'Product',
        'foreignKey' => 'component1_id'
    ),
    'ProductWithComponentAsComponent2' => array(
        'className' => 'Product',
        'foreignKey' => 'component2_id'
    )
);

Basically, you should replace your hasOne with hasMany. Each component hasMany products where it is the first component. At the same time, it hasMany products where it is the second component. Hope that makes things clear.

EDIT 1: (Oh, and "ProductWithComponentAsComponent#" is just for the purpose of explanation. You can keep whatever short, sweet alias you'd like depending on your actual models.)

EDIT 2: Simple rule of thumb for using hasOne relationships - Use it only when you're splitting a single table into many (like User/Profile)

EDIT 3: If you want all Products for a Component, then you can do this in two ways.

(A) Define the foreignKey as false in your hasMany relation.

var $hasMany = array(
    'Product' => array(
        'className' => 'Product',
        'foreignKey' => false,
        'conditions' => array(
            'or' => array(
                "Product.component1_id = Component.id",
                "Product.component2_id = Component.id"
            )
        )
    )
);

(B) If the above method does not work (Cake acts weird now and then), you can also use a join to make it obey. Create a function in the Component model like so:

function fetchProducts($id) {
    if (!$id) {
        return null;
    }
    return $this->Product->find('all', array(
        'fields' => array('Product.*'),
        'joins' => array(
            array(
                'table' => 'components',
                'alias' => 'Component',
                'foreignKey' => false,
                'type' => 'inner',
                'conditions' => array(
                    'or' => array(
                        "Product.component1_id = Component.id",
                        "Product.component2_id = Component.id"
                    ),
                    'Component.id' => $id
                )
            )
        )
    ));
}

Answer 2

Why would you define a twin like this?

A twin, first or second born is a person. The thing that associates them with a parent is the fact they are a "child" and their "DOB" is the same.

So would you not do something like:

Person -> ID, Name, Age, DOB, Parent_ID

The parent_ID is stored into the childs record, and the twin is determined by comparing all children under a parent for DOB?

Does this make it any easier to set up your cakePHP relationships?