Stubbing case statements with Mocha

I was happily mocking away with Mocha, then I passed a stub to a case statement at which point I was a little flummoxed.

The Ruby documentation clearly states that the ‘when’ in a ‘case’ statement uses the ‘===’ method for comparing the subject so I couldn’t work out why something like the following wasn’t working:

# Code
class A; end
 
def foo(instance)
  case instance
    when A : 'an A instance'
    else 'not an A instance'
  end
end
 
# Test
a = stub_everything
a.stubs(:===).with(A).returns(true)
assert_equal 'an A instance', foo(a)

So I had a little play around in irb and found the following:

class A; end
a = A.new
a === A #=> false
A === a #=> true

This revealed that I was actually stubbing the wrong side of the operator, I changed this to the following and voila!

# Code
class A; end
 
def foo(instance)
  case instance
    when A : 'an A instance'
    else 'not an A instance'
  end
end
 
# Test
a = stub_everything
A.stubs(:===).with(a).returns(true)
assert_equal 'an A instance', foo(a)

Looks obvious now but certainly wasn’t at the time!