Access CarrierWave image url using SQL in Rails migration - sql

Question

I'm working on a small app that uses CarrierWave for image uploading, but (for reasons that are a bit long and not really relevant) we have to stop using CarrierWave and start uploading images with our own custom requests to different cloud storage services.
But I still need to have the URLs to images that were already uploaded using CarrierWave, and I won't have access to the image_url method once I unmount the CarrierWave uploader, so I want to save all the urls to a new column on my model, and to have that migration be as optimized as possible.
I've searched for ways to access directly the url from the database (I mean, CarrierWave has to store that somewhere) but no luck so far.
I could do something like this (business_image is the column that the uploader is mounted on, image_url is the new column where I want to store the url):
def change
add_column :business_cards, :image_url, :string
BusinessCard.all.each do |bc|
bc.update(image_url: bc.business_image_url)
end
end
But that is obviously not optimized at all since it loads all business cards and updates them one by one. Is there a way to do it in a single SQL statement?
(I'm using Rails 5.1.4, PostgreSQL 10.3 and carrierWave 1.2.1)

Answer 1

One way would be too use batch function and update record in batches, you can use transaction so that if there are any errors the changes would be rolled back.
def change
add_column :business_cards, :image_url, :string
BusinessCard.find_each(batch_size: xxx) do |bc|
ActiveRecord::Base.transaction do
bc.update(image_url: bc.business_image_url)
rescue => e
raise ActiveRecord::Rollback
end
end
end
Hope it helps!!