How to stub any instances for a given class using Rspec Mocks

Question

The following code raises an error: undefined method 'any_instance' for String:Class

require 'rspec'

RSpec.configure do |config|
  config.mock_with :rspec
end

describe String do
  it 'stubs' do
    String.any_instance.stub(:foo).and_return(1)
    ''.foo.should eq(1)
  end
end

How can I include the Mocks module into the Class or Object class?

Solution 1

any_instance was recently added to rspec, so your example now works for me as it is with rspec 2.7.

Update for rspec 3:

The new way to do this is allow_any_instance_of(String).to receive(:foo).and_return(1)

Here is more any_instance documentation: https://relishapp.com/rspec/rspec-mocks/docs/working-with-legacy-code/any-instance

Solution 2

With RSpec Mocks in versions previous to 2.6.0, you cannot do it. However you can use any_instance with Mocha(as seen here) or in later versions of Rspec.

In your spec/spec_helper.rb

Make sure you have this line:

config.mock_with :mocha

uncommented.

Comments

[Rein Henrichs]

Well, given that the question is "using Rspec Mocks", I'm not sure the answer "don't use Rspec Mocks" is useful. Then again, it's better than the technically correct answer: you can't do it.

[Andy]

The test above is actually a test from Rspec itself link. And the module AnyInstance gets included through (mocks.rb): Class.class_eval { include RSpec::Mocks::AnyInstance } I think there is a way to do it somehow.

[David Chelimsky]

That is the current master but hasn't been released yet. It will be part of the rspec-mocks-2.6.0 release, within the next week or two.

[lulalala]

I am in 2.7.1 but have a similar issue too

[user650654]

Rspec Mocks 3.0 doc: https://www.relishapp.com/rspec/rspec-mocks/docs/working-wit‌​h-legacy-code/any-in‌​stance