Created: 2008-05-29 18:20
Updated: 2019-02-22 10:21
License: mit


Stubby - lightweight and fast stubbing

Stubby is a lightweight and fast stubbing framework that was

  • designed to help with the repetitive work of setting up stub scenarios for specifying/testing and
  • optimized for speed and ease of use.

Stubby does not track references to stubbed methods on real objects (like RSpec's stub! method does it which is used by stub_model & co).

Instead it dynamically creates a Ruby class for every stub object once. These classes will then be instantiated for every specification (or test). This circumvents the overhead of adding, tracking and removing lots of methods and can - depending on your stubs and specs - result in significant speed improvements.

Stubs setup

Stubby comes with a slick DSL to make it easy to describe stub setups. Basically there are three main statements:

  • define Model &block - defines a stub base class that camouflages as a model class
  • instance :key, methods - defines a concrete stub that inherits from a stub base class and will be instantiated on request
  • scenario :key &block - defines a code block that can be executed from your spec (e.g. in before :each blocks)

Things are pretty much readable and self-explaining:

define Site do
  methods :save => true, :destroy => true, :active? => true
  instance :homepage, :id => 1
  instance :customer, :id => 2, :active? => false

This creates a stub base class that camouflages as your Site model (i.e. it behaves as this model when methods like class, is_a?, === etc. are sent to it).

For the Site stub base class there are a couple of methods defined (stubbed) that you can call in your specs and that will return the given values (like save will return true).

It also defines two stub instances for this class that can be accessed through the keys :homepage and :customer. These both respond to the id method. The :customer instance additionally overwrites the active? value that's present in the base class.

Access stubs

You can access these stubs from

  • the stub base class definition block (like the one above)
  • scenario blocks (see below)
  • your specs

To access them you can use the following method apis. With the Site stub base class above being defined:

stub_site             # returns the first defined stub instance (i.e. :homepage)
stub_site(:homepage)  # returns the :homepage stub instance
stub_sites            # returns an array with all defined stub instances
stub_sites(:homepage) # returns an array containing the :homepage stub instance

You can also use the lookup(:site, :homepage) method in the same way in case you need it.

Also, note that you can use these accessors from within the stub base class definition, too:

define Site do
  instance :homepage, :next => stub_site(:customer)
  instance :customer, :next => stub_site(:homepage)

This creates a method next on both instances which return the respective stub instances.

For convenience you can pass arrays of method names as keys of method hashs:

define Site do
  methods [:save, :destroy, :active?] => true # same result as above

Stubbing ActiveRecord associations

To make stubbing of more complex ActiveRecord models easier there are also the following helpers:

define Site do
  has_many :sections, [:find, :build] => stub_section,
                       :paginate => stub_sections  

This has the expected effects. With this definition in place the following lines will now all return the same section stub object:


Watch the first of these lines! As you can see the sections has_many_proxy will automatically be populated with the array stub_sections if you don't specify anything explicitely. I.e. that's the same as:

define Site do
  has_many :sections, stub_sections,
                      [:find, :build] => stub_section,
                       :paginate => stub_sections  

If you want the sections array to contain something else you can specify it:

define Site do
  has_many :sections, stub_sections(:root),
                      [:find, :build] => stub_section,
                       :paginate => stub_sections  

Finally there are also the has_one and belongs_to statements which behave in the expected manner:

define Site do
  has_one :api_key
  belongs_to :user

The belongs_to statement implicitely defines the user_id method so that it returns the id defined for the stub_user stub.


A scenario is simply a code block that can be accessed and executed from your specs. You can define a scenario like this:

scenario :default do
  @site = stub_site
  Site.stub!(:find).and_return @site 

You can then invoke this scenario from your Spec like so:

before :each do
  scenario :default # pass as many scenario names as you like

Because the scenario code block is evaluated in the scope of the before :each block the @site instance variable is then accessible from within your specs.

Installation and Setup

Install Stubby as a usual Rails plugin and add the following line somewhere in your specs where it gets executed. The best place for this is probably your spec/spec_helper.rb file:


This requires the stubby code and loads the stub definition files once.

Stub definition files

Stubby assumes that you have on directory where you store your stub definition files (and nothing else). A stub definition file is just a ruby file that contains all or some of your stub base class and scenario definitions like described above. You can structure your files as you like.

By default Stubby assumes that you

  • use a directory named stubs/
  • that is located beneath the directory of your spec_helper.rb file
  • that starts requires the stubby code

Stubby then tries to guess the correct directory location. If it gets things wrong or if you want to use a different setup you can specify the directory where it looks for your stub definitions files like this: = 'path/to/stubs'


Be aware that Stubby is a framework for plain stubs. That means that it doesn't make a single attemp of looking at your model classes and trying to mimick their behaviour. This is different from what other solutions do (which might instantiate full functional model objects, like fixtures, or at least mimick the model's attributes or similar).

Thus you need to define every single method that your specs call. Personally I feel that this is an advantage because it makes visible what portions of the code my specs actually run (and what they shouldn't). But your mileage may vary.

Also, with Stubby it is currently not possible:

  • define class methods in stub base classes. You still need to stub class methods manually. The scenario block is a good place to stub common methods though.
  • stub methods in a way so that they return different results depending on a certain parameter passed. You can still use RSpec stub!(:method).with(:param).and_return(result) for that though.

If you want to help me with these, please let me know :)

Also, this code is brand new. Consider this an alpha release. I have written this library to clean-up and speed-up the 1000+ specs of a project that I'm currently working on. For me Stubby works quite well in this project's specs and it runs these specs in less than 30 seconds (which is less then 50% of what a solution with mock_model needed).

That doesn't mean that there aren't any major bugs or problems though. Please send me your pull requests, patches or just bug requests if so.

Similar solutions

There are a couple of similar solutions that aim at the same problem but all go down different routes. I highly recommend to have a look at:

The name

I was told that in Canada a "stubby" is a certain kind of beer bottle. The Wikipedia page lists a couple of advantages that perfectly match the reasons why I've written this library in the first place:

  • easier to handle
  • chills faster
  • less breakage
  • lighter in weight
  • less storage space
  • lower center of gravity

Now, given that I've thought of this name before I knew this page I think that's pretty funny and a perfect name for the library.

Also, I'm not a native English speaker but I've been informed on #rubyonrails that "Stubby" might also have some sexual connotations in some parts of the English speaking world.


Authors: Sven Fuchs
License: MIT

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