rake db:remove_orphans - detects orphans based on belongs_to relations and prompts to remove

gistfile1.rb

namespace :db do
  desc "Remove orphans"
  task :remove_orphans => :environment do
    Dir[Rails.root + "app/models/**/*.rb"].each do |path|
      require path
    end
    ActiveRecord::Base.send(:descendants).each do |model|
      model.reflections.each do |association_name, reflection|
        if reflection.macro == :belongs_to
          delete_all_like_this = false
          
          # Select only the id and the foreign key for this association
          select = ['id', reflection.foreign_key]

          # Also select the foreign type for this association if it is polymorphic
          select << reflection.foreign_type if reflection.options[:polymorphic]

          # Load all records and join the association we are checking to cut down on queries
          query = model
            .select(select.join(','))
            .includes(association_name)
            .where("#{reflection.foreign_key} IS NOT NULL")
          
          query.find_each do |model_instance|
            if model_instance.send(association_name).nil?
              puts model_instance.inspect

              if delete_all_like_this
                model_instance.destroy
              else
                puts "\n#{model.name}'s #{association_name} (id: #{model_instance.send(reflection.foreign_key)}) is nil, delete? [y/n/a]: "
                case STDIN.gets.strip
                  when "y", "Y"
                    model_instance.destroy
                  when "a", "A"
                    model_instance.destroy
                    delete_all_like_this = true
                end
              end
            end
          end
        end
      end
    end
  end
end