Today I Learned

26 posts about #rails

How to make path accessible in Rails Engine

Since routes in engine are isolated by default from main application so that you can have the same routes without clashing using default route helpers like about_path from main app will result in an error when you try to load it from inside the engine or access engine root from main app.

To fix that issue assuming you have an engine mounted:

Blorgh::Engine.routes.draw do
  resources :articles

to enforce using engine roots you need to change about_path to blorgh.about_path and to enforce using main application routes you need to change it to main_app.about_path

ActiveSupport::IncludeWithRange gotcha

Lets see how ruby implements === for ranges.

As documentation say “Returns true if obj is an element of the range, false otherwise”. Let’s try it out.

2.5.1 :001 > (1..10) === 5
 => true

Looks fine… how about if we compare it to another range?

 2.5.1 :001 > (1..10) === (5..15)
 => false

Seems to work properly again. How about if one range is a sub range of the other one.

2.5.1 :004 > (1..10) === (5..6)
 => false

As expected. Those ranges are not equal after all. Or at least (5..6) is not an element that (1..10) holds.

What is surprising, is what happens if we run the same thing in rails console (5.2.0 at the time of writing). Suddenly

[1] pry(main)> (1..10) === (5..6)
=> true

WAT? It now checks if range is included in the original range! Rails do not override === itself though. After looking at what rails adds to range…

[2] pry(main)> (1..10).class.ancestors
=> [ActiveSupport::EachTimeWithZone,

…we have identified this suspicious module ActiveSupport::IncludeWithRange. Its documentation explains everything.

# Extends the default Range#include? to support range comparisons.
#  (1..5).include?(1..5) # => true
#  (1..5).include?(2..3) # => true
#  (1..5).include?(2..6) # => false

Now guess what ruby’s Range#=== uses behind the scenes

              static VALUE
range_eqq(VALUE range, VALUE val)
    return rb_funcall(range, rb_intern("include?"), 1, val);

Yes… include?. The consequences are… there are consequences ;) The most annoying one is related to rspec.

expect(1..10).to match(5..6) # => true
expect([1..10]).to include(5..6) # => true
expect([1..10]).to match_array([5..6]) # => true

It is not possible to easily compare array of ranges matching on exact begin and end values yet disregarding actual order. Also the match behaviour is really misleading in my opinion. The only matcher we can use safely here is eq, as expect(1..10).to eq(5..6) will fail properly.

How to get XPath of Capybara's query

Have you ever found yourself in a situation, where you were trying to do something like e.g. click_link 'Approve' and Capybara was not able to find that element on the page despite the fact that it’s quite clearly visible, and you were asking yourself “what the heck is it searching for then?”. Or maybe your find(sth) is failing and you think it’s a bug in the Capybara 😱
Worry no more! Now you can easily check generated XPath used by Capybara*. In most cases, find(*args, **options) translates to:*args, session_options: current_scope.session_options, **options).xpath

E.g. to see XPath for click_on 'Approve':, 'Approve', session_options: current_scope.session_options).xpath

And XPath for find('tbody > tr > td:nth-child(2)'):'tbody > tr > td:nth-child(2)', session_options: current_scope.session_options).xpath

Then you can copy that XPath to the Chrome’s console and test it with $x('xpath').

* Presented solution doesn’t work with some types of more complicated queries, e.g. find('a', text: 'APPROVED') actually uses CSS selector instead of the XPath, and then filter results using Capybara::Result. You can check type of the selector used using .selector.format on your selector query.

Rake / rails console does not work in docker?

When using default ruby image in your Dockerfile (FROM ruby:2.5.1) if you encounter any problems with missing gems in your container when running rake task or rails console:

Could not find rake-x.y.z in any of the sources. Run bundle install to install missing gems.

That’s because you probably used:

RUN bundle install --deployment

You can fix it with:

RUN bundle install --without development test

Making enumerator from method that yields values

Original challenge related to AWS SQS QueuePoller

The challenge was to test a static method that yields multiple values but should stop, when some condition is met.

Let’s imagine such method as follows

class Poller
  def self.poll(condition = -> {})
    counter = 0

    while true do
      yield counter += 1
      break if

The problem is with testing such method. We do not only need to test what it yields, but we also need to test and control when it stops. To control when it stops, we need to access the actual block, but to test what it yields, we either need yield_successive_args matcher or we need to fetch consecutive results.

It is possible by aggregating each yielded value and then asserting them altogether, but the resultant code is not nice. The solution would be to make an Enumerator from the poll method and use next to get consecutive results. It is also easy as described in this blog post. The problem is, that we do not want to write code that is only required by our tests.

So the idea is to add creating enumerators when the block is not provided dynamically to the class when testing.

Poller.define_singleton_method(:poll_with_enum) do |*args, &block| 
    return enum_for(:poll) unless block.present?
    poll_without_enum(*args, &block)
# alias_method_chain is deprecated
# Poller.singleton_class.alias_method_chain(:poll, :enum)
Poller.singleton_class.alias_method :poll_without_enum, :poll
Poller.singleton_class.alias_method :poll, :poll_with_enum

if we turn this into a helper…

def with_enumerated(subject, method_name)
    subject.define_singleton_method("#{method_name}_with_enum") do |*args, &block|
      return enum_for(method_name, *args) unless block.present?
      public_send("#{method_name}_without_enum",*args, &block)

    subject.singleton_class.alias_method "#{method_name}_without_enum", method_name
    subject.singleton_class.alias_method method_name, "#{method_name}_with_enum"

    subject.singleton_class.alias_method method_name, "#{method_name}_without_enum"
    subject.singleton_class.remove_method "#{method_name}_with_enum"

…then we could leverage it in our tests!

with_enumerated(Poller, :poll) do
  $stop = false
  poller = Poller.poll(condition = -> { $stop == true })

  first_value =
  expect(first_value).to eq 1

  $stop = true
  second_value =
  expect(second_value).to eq 2

  expect { }.to raise_exception(StopIteration)

Handling uniqueness violations in factories

If it happens that your factories’ cross-dependencies result in creating records that violate uniqueness constraint, you can can fix it quick’n’dirty way

FactoryGirl.define do
  factory :day do
    initialize_with {
      Day.where(date: date).first_or_initialize

    sequence(:date) { |n| Date.current + n.days }

Ba aware that this is just a hack, and a hack that would add 1 SQL query to each creation of given object. Preferred way is to fix underlying problem. Still, if you do not care about retrieving the actual instance of object from factory, then different strategy can be used, that would mitigate extra query problem.

FactoryGirl.define do
  factory :day do
    to_create do |day|!
    rescue ActiveRecord::RecordNotUnique

    sequence(:date) { |n| Date.current + n.days }

If for any reason you really need to handle this kind of case, then introducing a custom create strategy (i.e. find_or_create(:day)) might be a way to go.

Mocking database views in Rspec

Sometimes it is a chore to feed database with all data we need for given database view to return results we are after within specs. Fortunately we can utilize temporary tables to mock such database views!

RSpec.configure do |config|
  ActiveRecord::Migration.verbose = false

  config.before(:all, :use_dummy_db_views) do |_example|
    ActiveRecord::Migration.create_table :my_database_view_entries, id: false, force: true do |t|
      t.integer :some_foreign_key      
      t.string :some_name
      t.string :some_code

    MyDatabaseViewModel.table_name = 'my_database_view_entries'

  config.after(:all, :use_dummy_db_views) do |_example|
    MyDatabaseViewModel.table_name = 'orignal_name_of_my_database_view'
    ActiveRecord::Migration.drop_table :my_database_view_entries, force: true    

Then it is just a matter of creating simulated view results, i.e. using factories.

# factory
FactoryBot.define do
  factory :my_database_view_model do
    some_foreign_key 10
    some_name 'Iron Man'
    some_code 'IRON'

# some spec
it 'some expectation', :use_dummy_db_views do
  # ...

Running heavy specs on CircleCI on demand

We have some specs that we do want to run only once a day (smoke specs that connect to actual live services). To handle this case we have introduced following setup:


--exclude-pattern "spec/smoke/*_spec.rb"


- run:
    name: run specs
    command: |
      if [[ ${RUN_ALL_TESTS} == "true" ]]; then
        bundle exec rspec --format progress --exclude-pattern ""
        bundle exec rspec --format progress

This way we can control if some specs are excluded or not, using ENV variable. You can then trigger such build on demand or use CircleCI Workflows.

How to run build on CircleCI recurrently

To run CircleCI build nightly (or at any interval we want), we need a couple of things. First we need to get CircleCI token that will allow us to access CircleCI API.

Next we need a script, that will trigger the build. In example below we also provide an extra build parameter RUN_ALL_TESTS that effectively allows us to conditionally run some specs we do not want to run during regular build.

namespace :ci do
  desc 'Runs build on CircleCI'
  task build: :environment do
    command = 'curl -X POST --header "Content-Type: application/json" ' \
      "--data '{\"build_parameters\": {\"RUN_ALL_TESTS\": \"true\"}}' " \
      '' \


Last thing is to schedule the build. Simplest way is to use Heroku Scheduler and just configure it to run rake ci:build command.

Edit: No longer valid! For Circle 2.0 we can use workflows for the same effect!

Making factory_bot work with read-only models

In one of our apps we need to ensure that all models are in read-only mode. Still, for testing purposes we need to be able to create instances of such models. Following code makes it possible.

# spec/models/application_record_spec.rb
require 'rails_helper'

RSpec.describe ApplicationRecord do
  it 'ensures all descendants are read-only' do
    Unit =

    expect( eq true
    expect { Unit.create! }.to raise_exception(ActiveRecord::ReadOnlyRecord)

  it 'allows creating objects using factories' do
    Unit =

    expect { read_only(:unit) }.to change { Unit.count }.by(1)
  it 'disallows updating objects' do
    Unit =
    unit = read_only(:unit)

    expect { unit.update_attributes!(name: 'New name') }.to \

# spec/support/factory_bot.rb
module FactoryBot
  module Strategy
    class ReadOnly <  Create
      def result(evaluation)
        is_readonly = evaluation.object.readonly?
        evaluation.object.define_singleton_method(:readonly?, -> { false })

        super.tap do |object|
          object.define_singleton_method(:readonly?, -> { is_readonly })

FactoryBot.register_strategy(:read_only, FactoryBot::Strategy::ReadOnly)

RSpec.configure do |config|
  config.include FactoryBot::Syntax::Methods

# spec/factories/units_factory.rb
FactoryBot.define do
  factory :unit do
    sequence(:name) { |i| "Unit #{i}" }

Configuring capybara-screenshot with Heroku

Due to many problems with our capybara-based automations we execute in Sidekiq on Heroku, we did need some visual feedback of what is going wrong. Unfortunately due to read-only nature of Heroku’s file system we did need to customize capybara-screenshot a bit to achieve this functionality.

# initializers/capybara_screenshot.rb

Capybara::Screenshot.s3_configuration = {
  s3_client_credentials: {
    access_key_id: ENV['DEBUG_BUCKET_S3_KEY'],
    secret_access_key: ENV['DEBUG_BUCKET_S3_SECRET'],
  bucket_name: ENV['DEBUG_BUCKET_S3_BUCKET']

# Default available methods use lunchy gem. 
# We do neither need nor want that. 
# Hence introducing simplified version.
Capybara::Screenshot.class_eval do
  def self.save_screenshot
    new_saver(Capybara,, false).save

Capybara.save_path = '/tmp' # Writeable directory on heroku

Then, we have decided to rescue and re-raise all exceptions, but also save a screenshot in the process…

rescue => exception # rubocop:disable Style/RescueStandardError
  raise exception

What to look for when your factory is missing id

So yesterday evening I’ve spotted a factory that was missing its _factory suffix. Easy peasy, quick rename and off we go… yes.. no… suddenly factories linting failed with following error

  The following factories are invalid:

  * lockoff - PG::NotNullViolation: ERROR:  null value in column "id" violates not-null constraint
  DETAIL:  Failing row contains (null, null, null, {}, {}, null, null, 2018-06-13 20:20:02.606616, 2018-06-13 20:20:02.606616).
  : INSERT INTO "lockoffs" ("created_at", "updated_at") VALUES ($1, $2) RETURNING "id" (ActiveRecord::StatementInvalid)

null value in column "id" ? What? How? After checking absolutely everything reasonable, I’ve checked column definitions. It turned out, that during selecting changes for structure.sql, somebody forgot to add one important piece to commit, that was effectively preventing automatic generation of consecutive primary key values.

-- Name: lockoffs id; Type: DEFAULT; Schema: public; Owner: -

ALTER TABLE ONLY lockoffs ALTER COLUMN id SET DEFAULT nextval('lockoffs_id_seq'::regclass);

If you ever face similar issue, do not forget to look for something like that as well…

Joining multiple change expectations in rspec

To look for multiple changes on multiple objects, we could aggregate those changes and verify them en-masse, i.e.

expect { }.to \
  change { [property_1, property_2].map{ |p| p.reload.synced_at }.
    to([, 1, 1, 16, 35),, 1, 1, 16, 35)])

This will pick partial changes though (if only one value changes), so it is not recommended. Another way is nesting expectations, i.e.

expect {
  expect { }.to \
    change { property_1.reload.synced_at }.
      to(, 1, 1, 16, 35))
}.to change { property_2.reload.synced_at }.
  to(, 1, 1, 16, 35))

This will work, but you will get only one failure at a time. Still it is useful for joining conflicting expectations (i.e. to with not_to). For non conflicting ones, following syntax is recommended:

# using .and
expect { }.to \
  change { property_1.reload.synced_at }.
    to(, 1, 1, 16, 35)).and \
  change { property_2.reload.synced_at }.
    to(, 1, 1, 16, 35))


# using &
expect { }.to \
  change { property_1.reload.synced_at }.
  to(, 1, 1, 16, 35)) &
  change { property_2.reload.synced_at }.
  to(, 1, 1, 16, 35))

Properly yielding responses in Rspec

We had some problem with configuring response that should be yielded to block… after a short investigation, here are outcomes

allow(collection).to receive(:each).and_yield([1, 2])

will yield array ([1, 2]) to one block variable

allow(collection).to receive(:each).and_yield(1, 2)

will yield two values (1, 2) to two block variables

allow(collection).to receive(:each).and_yield(1).and_yield(2)

will yield two consecutive values (1, 2) to one block variable, twice

Clicked link and still on the same page (capybara)

When writing some automation i’ve tried to navigate to next page using a link…

click_on '6366'

…but saving and showing screenshot resulted in the same page being displayed. The reason was simple, link’s target was a new window (or tab). One way to handle this, is to switch context of window. It is as easy as

switch_to_window(window_opened_by { click_on '6366' })

but if you do not need/want to play around with multiple window contexts, consider following solution

visit find_link('6366')['href']

This way you will stay with current window context :)

Custom validation error messages when using forms

Normally for rails model (i.e. Brand) you would do:

              invalid: "should contain only 'a-z', '0-9' or '-' but not as starting or ending character"

When using validations in forms you need a small tweak:

  • activerecord => activemodel
  • brand => brand_form (assuming your form is BrandForm)

Final translation:

              invalid: "should contain only 'a-z', '0-9' or '-' but not as starting or ending character"

Note: invalid is the key for validation type, full list here

Problems matching body params using stub_request?

Surprised that
  headers: { content_type: 'application/x-www-form-urlencoded' },
  body: { name: 'Tony', surname: 'Stark'}

is not matched by

stub_request(:post, '').
    headers: { 'Content-Type' => 'application/x-www-form-urlencoded' },
    body: { name: 'Tony', surname: 'Stark' }
  ).to_return(body: 'OK')


Remember, that HTTP Client does not send actually a hash, and stub_request does not know automagically which encoding you intended to use for your body params. You need to make it explicit, i.e.

stub_request(:post, '').
    headers: { 'Content-Type' => 'application/x-www-form-urlencoded' },
    body: URI.encode_www_form({ name: 'Tony', surname: 'Stark' })
  ).to_return(body: 'OK')

Missing your POST data in controller ?

So you can see JSON payload in inspector…

{days: [{date: "2018-05-18", unitPricing: {1: 285, 2: 285}},…]}

…but cannot see it in controller?

pry(#<Apis::V1::PricingBulkUpdatesController>)> params
=> <ActionController::Parameters {"controller"=>"apis/v1/pricing_bulk_updates", "action"=>"create"} permitted: false>

Remember to POST it with 'Content-Type': 'application/json' header and sell yourself a nice, juicy facepalm :)

Using have_selector with css and exact: option

Using have_selector with css selector and exact: option basically does not work.


expect(page).to_not have_content('a.hidden', exact: 'Push pricing to CB')


The :exact option only has an effect on queries using the XPath#is method. Using it with the query "div.hidden" has no effect.

We can use :contains("") css selector though!

expect(page).to_not have_content('a.hidden:contains("Push pricing to CB")')