Mutability strikes again at the worst moment possible

Full source for this example.

While refactoring code and running specs to validate the code was still working, I decided it was also a good moment to refactor the tests as well.

These tests were pretty old and didn’t follow some of the current practices for our test suite (use factories, rspec’s expect syntax and the like) and it was a good opportunity to move object creation logic to a single place, since the objects under test are really important for this specific app and these factories would replace a truckload of copied and pasted code.

The code went from being the full creation declared in many different before blocks like:

before do
  @user = User.create(
    first_name: "John",
    last_name: "Doe",
    options: { hometown: "Tokyo" }
  )
end
# various matchers

To be a single factory:

FactoryGirl.define do

  factory :user do
    first_name "John"
    last_name  "Doe"
    options hometown: "Tokyo"
  end

end

That was simply used with a let block at the spec:

describe User do

  let(:user) { create(:user) }

  it 'should allow setting more options' do
    user.options[:timezone] = "UTC"
    expect(user.options).to eq(hometown: "Tokyo", timezone: "UTC")
  end

  it 'should allow setting yet another option' do
    user.options[:state] = "The Shire"
    expect(user.options).to eq(hometown: "Tokyo", state: "The Shire")
  end

end

So far, so good, running specs one by one shows everything works after the spec refactoring. When I finally run all specs together, surprise, specs start to fail with this weird error:

1) User should allow setting yet another option
   Failure/Error: expect(user.options).to eq(hometown: "Tokyo", state: "The Shire")

     expected: {:hometown=>"Tokyo", :state=>"The Shire"}
          got: {:hometown=>"Tokyo", :timezone=>"UTC", :state=>"The Shire"}

     (compared using ==)

     Diff:
     @@ -1,3 +1,4 @@
      :hometown => "Tokyo",
      :state => "The Shire",
     +:timezone => "UTC",

How come this spec has the timezone field that was set at the other spec? Each spec get’s it’s own user reference created by the factory since we’re using the let block. There’s no way the same user would be reused by both specs.

So, what’s wrong here?

It had to me something I did. Before the refactoring, all specs were green, only after I refactored the user creation code to to live at the factory this started happening, my changes are causing this weird behavior.

Mutable objects at immutable factories

Since running specs one by one did work but running them all together didn’t (one spec was seeing state from the other) I had a shared state issue somewhere. Where could that be? It could be where it was being used or where it was being set.

Then I stared at the factory code again:

FactoryGirl.define do

  factory :user do
    first_name "John"
    last_name  "Doe"
    options hometown: "Tokyo"
  end

end

And BAM. There it was.

options hometown: "Tokyo"

The factory itself is immutable, it never changes after it was built when the code runs and this might have given me the impression that the values I declared there were immutable as well but they’re not.

To make it more visible, the factory could be written like this:

OPTIONS = {hometown: "Tokyo"}
FIRST_NAME = "John"
LAST_NAME = "Doe"

FactoryGirl.define do

  factory :user do
    first_name FIRST_NAME
    last_name  LAST_NAME
    options OPTIONS
  end

end

Now the issue is pretty visible, the hash given to the options attribute is created only once (when the factory is built) and it’s reused for all objects that are created out of that factory. So whenever I did a change at this hash it would be visible by all the other objects that were created from it as well.

And this isn’t just for hashes, any mutable object you declare at your factories, like strings (remember, strings are mutable in Ruby), arrays and the like could suffer from exactly the same effect.

For instance, to make it fail with strings you could use something like this:

it 'should change the full name if first name is changed' do
  user.first_name << "ny"
  expect(user.full_name).to eq("Johnny Doe")
end

it 'should change the full name of the last name is changed' do
  user.last_name << "rn"
  expect(user.full_name).to eq("John Doern")
end

Since the first spec mutates the actual string object, the second one will see Johnny Doern as a result. If you run the second one first, the first one will fail because the name will be Johny Doern instead of the expeted John Doe.

Expect the worst by default

Now when declaring factories for your objects and setting mutable values, it’s definitely simpler to go for the always create a new object solution, your factory doesn’t even have to change that much:

FactoryGirl.define do

  factory :user do
    first_name { "John" }
    last_name  { "Doe" }
    options do
      { hometown: "Tokyo" }
    end
  end

end

Using blocks for computing the values will guarantee that each object will get it’s own set of mutable objects and they will never be reused across different specs.

And with this the test suite runs and shows all tests to be green.

So, when using factories (and even when writing code in general):

After wasting some of my day trying to figure this out, I have definitely learned yet another lesson as to why I shouldn’t be mutating stuff.

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