Cannot re-assign images to another variant of the same product

[felixyz]

I have a problem concerning re-assigning images to another variant of the same product. Didn't have this problem in older versions of Solidus (but don't know when it came about).

Solidus Version:

2.11.0.alpha

To Reproduce

1. Go to /admin/products/my-product/images for a product with multiple variants and multiple images,
2. Re-assign an image from one variant to another
3. Press the "Save" icon

Current behavior

The update fails and the "The resource you were looking for could not be found" flash message is displayed. The image is not re-assigned.

Expected behavior

The update succeeds, the success flash message is displayed, and the image is re-assigned to the selected variant.

Desktop:

• OS: Ubuntu 20.04
• Browser: Firefox - Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser: Firefox 81.0

Additional context

The view for the images tab loads images using @product.gallery.images which will get all images from all of the product's variants (has_many :variant_images, -> { order(:position) }, source: :images, through: :variants_including_master).

But Spree::Api::ImagesController#update only tries to select the image for update with

@image = scope.images.accessible_by(current_ability, :update).find(params[:id])

where scope is a Spree::Product - in other words, images assigned to variants will not be found.