RSpec + Rubocop - 为什么 receive_message_chain 是代码异味？

Question

I am about to write specs for my custom validator, that uses this chain to check if a file attach with ActiveStorage is a txt:

return if blob.filename.extension.match?('txt')

Normally, I would be able to stub it with this call:

allow(attached_file).to receive_message_chain(:blob, :byte_size) { file_size }

Rubocop says it is an offence and points me to docs: https://www.rubydoc.info/gems/rubocop-rspec/1.7.0/RuboCop/Cop/RSpec/MessageChain

I would have to declare double for blob and byte_size and stub them in separate lines, ending up with 5 lines of code instead of 1. Am I missing something here?

Answer 1

Why should I avoid stubbing message chains?

I would have to declare double for blob and byte_size and stub them in separate lines, ending up with 5 lines of code instead of 1.

This is, in fact, the point. Having those 5 lines there likely will make you feel slightly uneasy. This can be thought of as positive design pressure . Your test setup being complex is telling you to have a look at the implementation. Using #receive_message_chains allows us to feel good about designs that expose complex interactions up front.

One of the authors of RSpec explains some of this in a GitHub issue .

What can I do instead?

One option is to attach a fixture file to the record in the setup phase of your test:

before do
  file_path = Rails.root.join("spec", "fixtures", "files", "text.txt")

  record.attribute.attach(io: File.open(file_path), filename: "text.txt")
end

This will test the validator end-to-end, without any stubbing.

---

Another option is to extract a named method, and then stub that instead.

In your validator:

def allowed_file_extension?
  blob.filename.extension.match?("txt")
end

In your test:

before do
  allow(validator).to receive(:allowed_file_extension?).and_return(true)
end

This has the added benefit of making the code a little clearer by naming a concept. (There's nothing preventing you from adding this method even if you use a test fixture.)