undefined method `marked_for_destruction?' CarrierWave, RMagick - carrierwave

Question

I am passing the ability to crop images, uploaded by Carrierwave. Here is RailsCast video on Youtube which I am following.
But after including RMagick in uploader, I received:
undefined method `marked_for_destruction?' for #<ImageUploader:0x007fe86634fcf0>
What a heck is this I thought. I haven't called this method anywhere. But if it is not defined, lets define it! And it worked! But later I checked more about this method and found that it is built in in Active Record Autosave Association module. And from docs, about this method:
Returns whether or not this record will be destroyed as part of the
parents save transaction.
Only useful if the :autosave option on the parent is enabled for this
associated model.
But I didn't passed autosave: true to any object!
So, my first question - was it done by default somehow?
2 - on RailsCast tutorial he didn't defined this method. Why I had to?
3 - I pass my code bellow. Any errors?
4 - if possible, could anyone explain how this process works, in general?
Many thanks!
product.rb:
has_one :image
validates :image, presence: true
mount_uploader :image, ImageUploader
products_controller.rb:
def create
#product = Product.new(product_params)
#product.category_id = params[:category_id]
#product.user_id = current_user.id
respond_to do |format|
if #product.save
if params[:product][:image].present?
format.html { render :crop }
else
format.html { redirect_to #product, notice: 'Product was successfully created.' }
format.json { render :show, status: :created, location: #product }
end
else
format.html { render :new }
format.json { render json: #product.errors, status: :unprocessable_entity }
end
end
end
image_uploader.rb:
class ImageUploader < CarrierWave::Uploader::Base
include CarrierWave::RMagick
def marked_for_destruction?
#marked_for_destruction
end
def mark_for_destruction
#marked_for_destruction = true
end
storage :file
def store_dir
"uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
end
version :large do
resize_to_limit(600,600)
end
end

Answer 1

If you're using Rails 5:
Open up new_framework_defaults.rb and change:
Rails.application.config.active_record.belongs_to_required_by_default = true
to
Rails.application.config.active_record.belongs_to_required_by_default = false
config.active_record.belongs_to_required_by_default is a boolean
value and controls whether a record fails validation if belongs_to
association is not present.

Answer 2

Inside your the *_uploader.rb file just write the function:
def marked_for_destruction?
end