Inter-Model Strategy

The purpose of our model classes is to house all of our state changing application logic. However a change to a model may be a trigger for other models to change as well. Consider these examples:

• A Transaction is added to a ledger. The Account balance needs to update.
• A User password is changed. An entry into the Log must be made.
• Stock adjustments are made when stock is received in. Orders waiting on the stock must be flagged as awaiting despatch.
• A Stock item is flagged as "Not For Sale". All Orders containing that stock must be marked "Held"

There are two types of inter model changes:

1. Updating 'children'

A model's children can be viewed as all other models related to this one as part of a one to many relationship. For example an Invoice model would have InvoiceLine models as children as part of the $Lines property. A Customer model would have Order models as children as part of the $Orders property.

In these cases the best practice is for the model to manipulate it's children directly as we can assume a parent knows all about their children.

class Invoice extends Model
{
    // ...
    public function applyDiscount( $percentage )
    {
        foreach( $this->Lines as $line )
        {
            $line->applyDiscount( $percentage );
        }
    }
    // ...
}

2. Updating a 'parent' or 'stranger'

A 'parent' is a model which can be navigated to through a one to one relationship. For example the parent of an InvoiceLine is an Invoice.

An 'stranger' is a model which is not navigable from the model via a relationship. For example the InvoiceLine model has no relationship with the Address model and so should not control it directly.

In both cases the event should be handled by an external agent that is listening for the event being raised on the ModelEventManager. The best candidate for this agent is the application's SolutionSchema object where the models and relationships are registered.

class MyDatabaseSchema extends SolutionSchema
{
    public function __construct()
    {
        parent::__construct( 0.1 );

        $this->AddModel( "GoodsReceived", "My\App\Model\Contracts\GoodsReceived" );
        $this->AddModel( "Contract", "My\App\Model\Contracts\Contract" );

        ModelEventManager::AttachEventHandler( "GoodsReceived", "AfterSave", function( $goodsReceived )
        {
            // Update the contract totals as a goods received has been adjusted.
            $goodsReceived->Contract->UpdateTotals();
        });
    }
}

The solution schema knows best how the models inter-relate and so can be seen as good choice of co-ordinator for inter-model events.