Rails 5.2 introduced ActiveStorage which made it possible to easily upload files to a cloud storage service like Amazon S3, Google Cloud Storage, or Microsoft Azure Storage. It also helped in attaching the files to active record objects.

Using ActiveStorage and ImageMagick we can transform image uploads and extract metadata from files. For transforming image uploads we can use image_processing gem with ActiveStorage and create variants of an image.

Previously for creating image variants, we needed to use image_processing gem with ActiveStorage processor. The default processor for ActiveStorage is MiniMagick, but we can also use Vips.

However, Rails 7 has added the ability to use pre-defined variants which provides a way to easily create variants for images.

Let's assume we have a model Blog. Using has_one_attched every record can have one file attached to it.

# app/models/blog.rb
class Blog < ApplicationRecord
  has_one_attached :display_picture # Setup mapping between record and file
end

To create a blog with an attachment on display_picture.

# app/views/blogs/new.html.erb
<%= form.file_field :avatar %>

# app/controllers/blogs_controller.rb
class BlogsController < ApplicationController
  def create
    blog = Blog.create!(blog_params)
    session[:user_id] = blog.id
    redirect_to root_path
  end

  private
    def blog_params
      params.require(:blog).permit(:title, :display_picture)
    end
end

Before

If we want to create variants of display_picture, we needed add the image_processing gem to the Gemfile.

# project_folder/Gemfile
gem 'image_processing'

Then to create variants of the image, we can call the variant method on the attachment record.

# app/views/blogs/show.html.erb
<%= image_tag blog.display_picture.variant(resize_to_limit: [100, 100]) %>

Rails 7 onwards

We can use the variants option on has_one_attached.

class Blog < ActiveRecord::Base
  has_one_attached :display_picture, variants: {
    thumb: { resize: "100x100" },
    medium: { resize: "300x300" }
  }
end

To display we can use the variant method.

# app/views/blogs/show.html.erb
<%= image_tag blog.display_picture.variant(:thumb) %>

variants can also be used on has_many_attached. Check out this pull request for more details.