[rspec-users] When to stub_model?

Matt Wynne matt at mattwynne.net
Tue May 19 13:11:40 EDT 2009

On 19 May 2009, at 14:53, Lee Longmore wrote:

> I am trying to specify a controller with RSpec for the first time.
> I have been using mock_model to date but, given that the models are  
> fully implemented, I am wondering if I should be using stub_model.
> Based on what I have read, I am struggling to understand the  
> advantages that stub_model over mock_model might provide.
> Should I be using stub_model and if so, why?

*Should* is a big word here. It depends.

When you use a stub model, you get a real instance of your AR model  
object, with the database connection crippled so you can't  
accidentally do something that will make your specs slow. The benefit  
of this is that you get all the methods on the model available to the  
controller. For example, if you have a User class which takes a  
date_of_birth attribute in it's constructor, you can call the #age  

This can make your tests easier to read.

The cost of this is that your controller specs will be covering code  
in the model classes as well as the controller you're explicitly  
testing. There is a chance you'll make a change to the #age method on  
your User (maybe you want to specify in days rather than years), make  
the user specs pass, and check in believing you're done. Unwittingly,  
you have just broken the controller specs too.

If you read what from the really experienced guys who started this  
stuff off[1] say, you'll hear that mocks are really a design tool. By  
creating a mock_model in your controller specs rather than leaning on  
the stub_model, you're designing your 'ideal model', rather than being  
constrained by whatever default methods activerecord gives you, or you  
have already written. I often find that this process drives out much  
more elegant interfaces onto the models, and sometimes even shows me  
where I need to introduce an intermediate class.

I think this is something you really have to play around with for  
yourself to find the right balance for you and your team.

Matt Wynne

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