product.rb

  def update_cousin_variants_by_variant_value(variant_value)
    first_sibling_value = variant_value.category.variant_values.first
    existing_variants_with_this_value = self.variants.includes(:variant_values).where('variant_value_id = ?', first_sibling_value.id)

    new_variants = []
    existing_variants_with_this_value.each do |variant|
      new_variant = product.variant.create(price: self.price)
      variant.variant_values.each do |old_variant_value|
        if old_variant_value.variant_category != variant_value.variant_category
          new_variant.variant_values << old_variant_value
        else
          new_variant.variant_values << variant_value
        end
      end
      new_variants << new_variant
    end

    new_variants
  end