Attachinary Setup

First add the following gems to your Gemfile:

# Gemfile
gem "attachinary"
gem "jquery-fileupload-rails"
gem "coffee-rails"

Then open the terminal and launch:

bundle install
rails attachinary:install:migrations
rails db:migrate

Open your config/application.rb and add this line after all the require:

require "attachinary/orm/active_record"

Open the config/routes.rb file and add this as first line in the draw block:

mount Attachinary::Engine => "/attachinary"

Open app/views/layout/application.html.erb and append this line after the main javascript_include_tag:

<%= cloudinary_js_config %>

Open app/assets/javascripts/application.js and append these lines before the require_tree:

//= require jquery-fileupload/basic
//= require cloudinary/jquery.cloudinary
//= require attachinary

Create a file app/assets/javascripts/init_attachinary.js and copy-paste those lines:

$(document).ready(function() {
  $('.attachinary-input').attachinary();
});

Usage

One picture per model

You need to update the model:

class Product < ApplicationRecord
  has_attachment :photo
  
  # [...]
end

And the form (simple_form gem used):

<%= f.input :photo, as: :attachinary %>

And the controller for strong params:

def product_params
  params.require(:product).permit(:name, :description, :photo)
end

To display the product photo in the view, add:

<% if @product.photo? %>
  <%= cl_image_tag @product.photo.path %>
<% end %>

Multiple pictures per model

You need to update the model:

class Product < ApplicationRecord
  has_attachments :photos, maximum: 2  # Be carefule with `s`
  
  # [...]
end

And the form (simple_form gem used):

<%= f.input :photos, as: :attachinary %>

And the controller for strong params:

def product_params
  params.require(:product).permit(:name, :description, photos: [])
end

To display the product photos in the view, add:

<% @product.photos.each do |photo| %>
  <%= cl_image_tag photo.path %>
<% end %>

Bonus - Devise

If you want to add an avatar to the User model, you need to sanitize regarding the strong params:

# app/controllers/application_controller
class ApplicationController < ActionController::Base
  before_action :configure_permitted_parameters, if: :devise_controller?

  def configure_permitted_parameters
    devise_parameter_sanitizer.permit(:sign_up,        keys: [:avatar])
    devise_parameter_sanitizer.permit(:account_update, keys: [:avatar])
  end
end