How to stub/mock without coupling the test to the code under test with rspec?

Question

Say there's a worker, whose job is to:

• find or create a record by a set of critaria;
• update an attribute of the record.

Here's a sample implementation:

class HardWorker
  include SidekiqWorker

  def perform(foo_id, bar_id)
    record = find_or_create(foo_id, bar_id)

    update_record(record)
  end

  private

  def find_or_create(foo_id, bar_id)
    MyRecord.find_or_create_by(foo_id: foo_id, bar_id: bar_id)
  end

  def update_record(record)
    result_of_complicated_calculations = ComplicatedService.new(record).call

    record.update(attribute: result_of_complicated_calculations)
  end
end

I want to test that:

• the worker creates the record if the record doesn't exist;
• the worker doesn't create a new record, but fetches the existing one if the record exists;
• in any case, the worker updates the record

One way to test the last would be to use expect_any_instance_of

expect_any_instance_of(MyRecord).to receive(:update)

The problem is that the usage of expect/allow_any_instance_of is discouraged:

The rspec-mocks API is designed for individual object instances, but this feature operates on entire classes of objects. As a result there are some semantically confusing edge cases. For example in expect_any_instance_of(Widget).to receive(:name).twice it isn't clear whether each specific instance is expected to receive name twice, or if two receives total are expected. (It's the former.)

Using this feature is often a design smell. It may be that your test is trying to do too much or that the object under test is too complex.

It is the most complicated feature of rspec-mocks, and has historically received the most bug reports. (None of the core team actively use it, which doesn't help.)

The proper way would be to use an instance_double. So I would try:

record = instance_double('record')

expect(MyRecord).to receive(:find_or_create_by).and_return(record)

expect(record).to receive(:update!)

This is all nice and fine, however, what if I have this implementation:

MyRecord.includes(:foo, :bar).find_or_create_by(foo_id: foo_id, bar_id: bar_id)

Now, expect(MyRecord).to receive(:find_or_create_by).and_return(record), won't work, because actually the object, that receives find_or_create_by is an instance of MyRecord::ActiveRecord_Relation.

So now I need to stub the call to includes:

record = instance_double('record')

relation = instance_double('acitve_record_relation')

expect(MyRecord).to receive(:includes).and_return(relation)

expect(relation).to receive(:find_or_create_by).and_return(record)

Also, say I call my service like so:

ComplicatedService.new(record.baz, record.dam).call

Now, I'll get errors that unexpected messages baz and dam were received by record. Now I need to either expect/allow those messages or use a Null object double.

So after all this, I end up with a test, which super tightly reflects the implementation of the methods/classes that are under test. Why should I care, that some additional records are eager-loaded via includes, while fetching the record? Why should I care, that before calling update, I also call some methods (baz, dam) on the record?

Is this a limitation of the rspec-mocks framework / the philosophy of the framework or am I using it wrong?

Answer 1

I modified the initial version a bit to make it easier to test:

class HardWorker
  include SidekiqWorker

  def perform(foo_id, bar_id)
    record = find_or_create(foo_id, bar_id)

    update_record(record)
  end

  private

  def find_or_create(foo_id, bar_id)
    MyRecord.find_or_create_by(foo_id: foo_id, bar_id: bar_id)
  end

  def update_record(record)
    # change for easier stubbing
    result_of_complicated_calculations = ComplicatedService.call(record)

    record.update(attribute: result_of_complicated_calculations)
  end
end

The way, I would test this is:

describe HardWorker do
  before do
    # stub once and return an "unique value"
    allow(ComplicatedService).to receive(:call).with(instance_of(HardWorker)).and_return :result_from_service
  end

  # then do two simple tests
  it 'creates new record when one does not exists' do
    allow(ComplicatedService).to receive(:call).with(instance_of(HardWorker)).and_return :result_from_service
    HardWorker.call(1, 2)

    record = MyRecord.find(foo_id: 1, bar_id: 2)

    expect(record.attribute).to eq :result_from_service
  end

  it 'updates existing record when one exists' do
    record = create foo_id: 1, bar_id: 2

    HardWorker.call(record.foo_id, record.bar_id)

    record.reload

    expect(record.attribute).to eq :result_from_service
  end
end