NoMethodError: undefined method `attached?' for <#Attachment> with nested records

[marckohlbrugge]

# promotion.rb
class Promotion
  belongs_to :product, optional: true
  has_one_attached :image
  validates :image, processable_image: true
end

# product.rb
class Product
  has_many :attachments
end

# attachment.rb
class Attachment
  belongs_to :product
  has_one_attached :image
  # no validations
end

Running the following code returns an error:

product = Product.create! …
attachment = Attachment.create! product_id: product, …

promotion = Promotion.create! product: product
=> NoMethodError: undefined method `attached?' for #<Attachment …

Unfortunately the stack trace is useless as it leads to method_missing in active_model/attribute_methods.rb

There's two surprising things happening here:

1. Somehow the validation inside promotion.rb ends up calling .attached? on Attachment which is just a regular ActiveRecord model. It should be calling it on the image instead.
2. Removing that validation from promotion.rb resolves the issue.

Any idea what could be going on here?