# How to mock a model method for unit tests?

**URL:** <https://www.ruby-forum.com/t/how-to-mock-a-model-method-for-unit-tests/63486>\
**Category:** Rails\
**Created:** [June 15, 2006, 5:21pm UTC](https://www.ruby-forum.com/t/how-to-mock-a-model-method-for-unit-tests/63486 "2006-06-15T17:21:57Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![jeff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e19b73/32.png) [@jeff](https://www.ruby-forum.com/u/jeff)\
**Post date:** [June 15, 2006, 5:21pm UTC](https://www.ruby-forum.com/t/how-to-mock-a-model-method-for-unit-tests/63486/1 "2006-06-15T17:21:57Z")

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I have a model class, say, model.rb, and in the normal course of things  
it creates an instance of another class, say utility.rb (also in the  
models directory).

class Utility

def initialize(obj)  
# do stuff…  
end

def do\_something\_expensive  
# lots of stuff here  
end

end

During testing, I want to mock out do\_something\_expensive because it’s  
not important for the tests, but only during development. So I went to  
the tests\mocks folder and added a file called utility.rb with only  
this:

# this is the mock file

class Utility

def do\_something\_expensive  
# shortcut, just return true to satisfy client code  
true  
end

end

So this way, in my tests, when my model uses the Utility class, it  
should be insantiating my mock instead.

But when I rake test:units, I get a syntax error saying that it can’t  
construct a Utility object with the original constructor: “Wrong number  
of argumes (0 for 1)”.

It’s as if the mock class is totally taking over, whereas I just want to  
override the do\_something\_expensive and leave everything else.

What am I doing wrong?

The Utility class is actually larger than I’m showing here, and I’m  
finding I have to basically copy all of the methods into the mock to get  
it to work.

Jeff

---

<div class="post-metadata">

**Author:** ![jeff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e19b73/32.png) [@jeff](https://www.ruby-forum.com/u/jeff)\
**Post date:** [June 15, 2006, 5:42pm UTC](https://www.ruby-forum.com/t/how-to-mock-a-model-method-for-unit-tests/63486/2 "2006-06-15T17:42:54Z")

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Jeff C. wrote:

> But when I rake test:units, I get a syntax error saying that it can’t  
> construct a Utility object with the original constructor: “Wrong number  
> of argumes (0 for 1)”.
> 
> It’s as if the mock class is totally taking over, whereas I just want to  
> override the do\_something\_expensive and leave everything else.
> 
> What am I doing wrong?

You need to require the original class so that you’re re-opening it, as  
opposed to redefining it from scratch. Then you can knock out the one  
unwanted function, while leaving all the others intack

Eric

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<div class="post-metadata">

**Author:** ![jeff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e19b73/32.png) [@jeff](https://www.ruby-forum.com/u/jeff)\
**Post date:** [June 15, 2006, 5:59pm UTC](https://www.ruby-forum.com/t/how-to-mock-a-model-method-for-unit-tests/63486/3 "2006-06-15T17:59:13Z")

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On Thursday, June 15, 2006, at 5:42 PM, Eric D. Nielsen wrote:

> You need to require the original class so that you’re re-opening it, as  
> [http://lists.rubyonrails.org/mailman/listinfo/rails](http://lists.rubyonrails.org/mailman/listinfo/rails)  
> you should also put the mocks in the ‘test/mock/development’ or ‘test/  
> mock/test’ if you want it to only load during development or testing.

\_Kevin

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<div class="post-metadata">

**Author:** ![jeff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e19b73/32.png) [@jeff](https://www.ruby-forum.com/u/jeff)\
**Post date:** [June 15, 2006, 6:25pm UTC](https://www.ruby-forum.com/t/how-to-mock-a-model-method-for-unit-tests/63486/4 "2006-06-15T18:25:38Z")

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On Thursday, June 15, 2006, at 5:42 PM, Eric D. Nielsen wrote:

> > You need to require the original class so that you’re re-opening it, as  
> > [http://lists.rubyonrails.org/mailman/listinfo/rails](http://lists.rubyonrails.org/mailman/listinfo/rails)

Doh!! Thanks a lot, Eric.

Kevin O. wrote:

> you should also put the mocks in the ‘test/mock/development’ or ‘test/  
> mock/test’ if you want it to only load during development or testing.
> 
> \_Kevin

Yes, I forgot to say that I did put it in the test/ folder, but thanks  
for pointing that out.

Jeff