How can I relate a FK back to the purchasable ID?
Thanks Brad for breaking it down like that. It helped me take a step back and see what it was I was doing.

Here is the relationship I have that is working:

public function defineAttributes()
{
    return array(
        'id' =&gt; AttributeType::Number,
        'packageId' =&gt; AttributeType::Number,
        ...
    );
}


public function defineRelations()
{
    return array(
        'element' =&gt; array(static::BELONGS_TO, 'ElementRecord', 'id', 'required' =&gt; true, 'onDelete' =&gt; static::CASCADE),
        'package' =&gt; array(static::BELONGS_TO, 'Commerce_PurchasableRecord', 'onDelete' =&gt; static::CASCADE),
    );
}


Now I am able to have a relation to the user id element and back to my product package.