Stub all instances

gistfile1.rb

# Create a class method (anyone, anywhere)
User.stub :save
# Save original constructor
new_method = User.method(:new)
# Stub constructor
User.stub(:new).and_return do |*args|
  # Call original constructor
  instance = new_method.call(*args)
  # Save original save method
  save_method = instance.method(:save)
  # Stub save method
  instance.stub(:save).and_return do |*args|
    # Call newly created class method 
    User.save(*args)
    # Call original save method and return it
    save_method.call(*args)
  end
  # Return new instance
  instance
end

# User.any_instance.should_receive(:save).at_least(:once)
User.should_receive(:save).at_least(:once)

Inspired by https://gist.github.com/chischaschos/853602
To fix http://stackoverflow.com/q/9800992