RSpec any_instance return self

Question

I'm trying to stub any instance of some class. I need to stub the fetch method, which fills the self with some data.

How can I get access to self variable, modify it and return on fetch method?

MyObject.any_instance.stub(:fetch) { self }

doesn't return a MyObject instance.

Maybe, mocks is more useful in this situation. Unfortunately, I haven't understood they yet.

Answer 1

There's an open rspec-mocks issue to address this. I hope to get around to addressing it at some point, but it's not simple to add this in a way that doesn't break existing spec suites that use any_instance with a block implementation, because we would start yielding an additional argument (e.g. the object instance).

Overall, any_instance can come in handy in some situations, but it's a bit of a smell, and you'll generally have fewer issues if you can find a way to mock or stub individual instances.

Here's a work around that I have not tested but should work:

orig_new = MyObject.method(:new)
MyObject.stub(:new) do |*args, &block|
  orig_new.call(*args, &block).tap do |instance|
    instance.stub(:fetch) { instance }
  end
end

Essentially, we're simulating any_instance here by hooking into MyObject.new so that we can stub fetch on each new instance that is instantiated.

All that said, it's important to "listen to your tests", and, when something is hard to test, consider what that says about your design, rather than immediately using power tools like any_instance. Your original question doesn't give enough context for me to speculate anything about your design, but it's definitely where I would start when faced with a need to do this.