FuelPHP ORM Primary Key on model cannot be changed - fuelphp

Question

I've been banging my head with this ORM error:
Fuel\Core\FuelException [ Error ]: Primary key on model Model_CustomValue cannot be changed.
Here are relevant info from my models I'm having issues with:
<?php
use Orm\Model;
class Model_Purchase extends Model
{
protected static $_has_many = array(
'customvalues' => array(
'model_to' => 'Model_CustomValue',
'key_to' => 'purchase_id',
'cascade_delete' => true,
)
);
protected static $_properties = array(
'id',
'customer_id',
'payment_id',
'audit_id',
'created_at',
'updated_at',
);
<?php
use Orm\Model;
class Model_CustomValue extends Model
{
protected static $_table_name = 'customvalues';
protected static $_primary_key = array('purchase_id', 'customfield_id');
protected static $_belongs_to = array(
'purchase' => array(
'key_from' => 'purchase_id',
'model_to' => 'Model_Purchase',
'key_to' => 'id',
),
);
When trying to save the Model_Purchase with an array of Model_CustomValue objects as a property named 'customvalues' on the $purchase object, I get the "Primary key on model Model_CustomValue cannot be changed."
I've tried swapping the key_from/to in the "belongs_to" on the Model_CustomValue, but to no avail.
I'm using Fuel 1.6 (hash: 6e6d764)
Please let me know if more information would be helpful, and I'll provide.

Answer 1

From the FuelPHP forum thread, Harro answered:
You can not have a column which is at the same time FK and PK. Which
you have on your Model_CustomValue.
The reason for that is that when you disconnect a relation, the FK
will be set to NULL, which should not happen with a PK.
I then clarified, for those of us who may need specific examples from the original example, I confirmed the following:
So just re-stating why that's not allowed:
Model_CustomValue uses the "purchase_id" as part of its PK as well as the FK to Model_Purchase. And if the two Models were to be unlinked, that would lead to a null portion of the PK for Model_CustomValue -- which obviously isn't allowed.