Override a private method in the lib folder of a gem

Question

There is a private method in the spree-auth-devise gem. The method is inside the controller UserSessionsController https://github.com/spree/spree_auth_devise/blob/master/lib/controllers/frontend/spree/user_sessions_controller.rb

I wish to override the function "redirect_back_or_default".

Is this possible?

Update

After mixing and matching between your answers and doing some googling, I arrived at this solution:

    Spree::UserSessionsController.class_eval do
        private
        def redirect_back_or_default(default)
          if default == "/admin/orders" or session["spree_user_return_to"] == "/admin/orders"
            redirect_to("/admin/users")
          else
            redirect_to(session["spree_user_return_to"] || default)
          end
            session["spree_user_return_to"] = nil
        end
    end

And I have placed the script file in config/initializers.

Thank you all.

Answer 1

You could do something like this:

class HelloWorld
  def run
    say_hello_world
  end

  private 
    def say_hello_world
      puts "hello world"
    end
end

HelloWorld.new.run 
"hello world"
=> nil

Now let's extend/override the current behaviour.

class HelloWorld 
  private
    def say_hello_world
      puts "Goodbye"
    end 
end

HelloWorld.new.run
"Goodbye"
=> nil

So since this works and is possible, how about monkey patching it. Something like this:

class Spree::UserSessionsController < Devise::SessionsController
  private
    def redirect_back_or_default(default)
      # your logic
    end
end

Answer 2

Of course you can override private methods:

class Super
  private def i_am_private
    'private method'
  end
end

class Sub < Super
  private def i_am_private
    'overridden ' << super
  end
end

Sub.new.send(:i_am_private)
# => 'overridden private method'