original_relations populated with non-existent IDs of new Models

[andrewwheal]

I have Model_Product which has_many Model_Images and relates to Model_Category in a many_to_many

When editing a Model_Product the original_relations entry for it's Model_Images is getting set to an array of empty strings.

$product = Model_Product::find(1);

// $product->images == [ 1, 2, 3, 4, 5, 6 ]

unset($product->images);
$product->image = Model_Image::query()->where('id', 'IN', [ 20, 21, 22 ])->get();

...

// Within another method which is only passed the product's id
Model_Category::query()
    ->related('products')
    ->where('products.id', $product_id)
    ->get();

...

// $product->_original_relations['images'] == [ '', '', '' ]

$product->save();

// $product->images == [ 1, 2, 3, 4, 5, 6, 20, 21, 22 ]

After stepping through the execution of ORM I have isolated the problem to be the call to Orm\Model::_update_original_relations() from Orm\Query::hydrate() which is run as part of executing the Model_Category::query().

I think that #312 might solve this issue in my instance because the relations are already loaded when Orm\Query::hydrate() is run. However, in a more general case I think it might be necessary to check within Orm\Model::_update_original_relations() that the data the "original" IDs are being retrieved from is not new.

[andrewwheal]

Added a pull request (#328) which solves this for me, not sure if it is the best solution but stops the above from being able to happen.