Rails + Sidekiq::Web: Configuration for wildcard session cookies

When you're using Sidekiq::Web to monitor the Sidekiq status AND have your session cookie configured to a wildcard domain like, you need to take an additional step to keep your cookies valid.


Sidekiq::Web is mounted into your Rails application and will use the Rails session cookie for protection from CSRF attacs. While it somehow figures out the cookie name, it does NOT respect cookie configuration like a custo…


Reading an element's attributes with Capybara

capybara_element['attribute_name'] allows accessing an element's attributes in Capybara.

A few examples:

# => "first-class second-class"

# => "My placeholder value"

# => ""

# => nil

Fix for: "can't convert nil to String" when running "gem update --system"

When attempting to update RubyGems, depending on updates your previously performed, you might run into an error

ERROR:  While executing gem ... (TypeError)
can't convert nil into String

Fix this by running

gem uninstall rubygems-update -a -x

Using Rails with Webpack: First steps

Webpack opens the world of npm to Rails developers. It does not collide with the asset pipeline—however, it's kind of a parallel world and you'll probably be better off going for either Webpack or asset pipeline.


  1. Install a recent Node.js

    curl -sL | sudo -E bash -
    sudo apt-get install -y nodejs
  2. Install Yarn

    curl -sS

MySQL will not use indexes if you query the wrong data type

When MySQL refuses to use your index, you can do a number of things wrong. One of them might be conditions with improper data types.

An example

For example, let's assume you have a users table with an email field (varchar) which is indexed.

MySQL will use the index when your query is well-formed:

mysql> EXPLAIN SELECT * FROM users WHERE email = '';
| id | select_type | table | type  | poss...

Ruby: How to make an object that works with multiple assignment

Ruby allows multiple assignment:

a, b, c = o

In order to prove multiple values from a single object, Ruby calls #to_a on the object:

o ='O')
def o.to_a

a, b, c = o # Implicit call to #to_a here
a # => 1
b # => 2
c # => 3

Hence, if you want your class to support multiple assignment, make sure to define a #to_a method.


RSpec: Tagging examples and example groups

In RSpec you can tag examples or example groups with any tags you like simply by saying

describe ReportCreator, slow: true do
  # ..

describe ReportCreator do
  it 'generates reports', slow: true do
    # ...

You can then only run examples with these tags.

rspec --tag slow
rspec -t slow

# Using the parallel_tests gem
rake "parallel:spec[,,--tag slow]"

Or you can run all examples except the ones with a certain tag:

rspec –tag ~slow # note the ~
rspec -t ~slow

Using the parallel_tests gem

rake "par…


Advisory: Excel converts CSV entries to formulas

If your application exports CSV, be advised that Excel and other spreadsheet applications treat certain cells (those starting with =, +, - or @ as formulas.

This is an issue if you output user input. Not only is it probably not what you want, it also poses a security risk. See the link for attack vectors.

Note that current Excel versions will warn the user when opening the file. At least for the code execution vulnerability, this warning seems adequate to me.


If possible, always **try to avoid CSV and output XLS(X) f…

How to install packages from newer Ubuntu releases

We're usually running Ubuntu LTS versions. Sometimes newer hardware requires packages from more recent Ubuntu releases that only come with 6 months of support. If there is really no other way, it's possible to install packages from later Ubuntu releases

Caution: Pay really close attention to what you're doing. Depending on the package, this process may require upgrading a lot of dependencies, possibly breaking the system! You really should not do this unless you've carefully calculated the impact on your system




BigDecimal arithmetic in Ruby

Ruby comes with a class BigDecimal which you can use for arbitrary precision arithmetic. You should use BigDecimal instead of Float whenever you care about rounding errors, e.g. whenever you are dealing with money.

You should remember these two rules when working with BigDecimal values:

  • When you add or multiply a BigDecimal with another BigDecimal, the …

JavaScript: Testing the type of a value

Checking if a JavaScript value is of a given type can be very confusing:

  • There are two operators typeof and instanceof which work very differently.
  • JavaScript has some primitive types, like string literals, that are not objects (as opposed to Ruby, where every value is an object).
  • Some values are sometimes are primitive value (e.g. "foo") and sometimes an object (new String("foo")) and each form requires different checks
  • There are three different types for null (null, undefined and NaN) and each has different rules f…

Capybara: Easiest way to get the parent of an element

If you already selected an element and want to get its parent, you can call find(:xpath, '..') on it.
To get the parents parent, call find(:xpath, '../..').


Find a link which contains a twitter icon and check that it links to the correct page:

<a href="">
  <i class="icon is-twitter"></i>
link = page.find("a").find(:xpath, '..')
link[:href].should == ""

There is a good overview on xpath syntax on [w3schools](…

External contentRepeats

Sidekiq: Problems and Troubleshooting

When using Sidekiq in your application, you must write thread-safe code.

This wiki page also lists gems that are known to be unsafe on threaded applications.
When adding a gem that will also be used by a Sidekiq worker, make sure to confirm it's thread-safe.

Webservice to test and inspect requests is a webservice that gives you a temporary URL you can use to test request. The page will automatically record and display the latest web request made to it.


Don't require files in random order

A common pattern in Ruby is to to require all files in a specific diretory, using something like

Dir.glob(Rails.root.join('lib/ext/**/*.rb')).each do |filename|
  require filename

However, this causes files to be required in an order determined by the file system. Since load order can be important, this may lead to different behavior on different machines which are hard to debug.

Simply add a .sort:

Dir.glob(Rails.root.join('lib/ext/**/*.rb')).sort.each do |filename|
  require filename

Selenium: How to close another tab (popup)

If you open a pop-up window [1] in your Selenium tests and you want to close it, you can do this:

# Find our target window
handle = page.driver.find_window("My window title")

# Close it

# Have the Selenium driver point to another window
last_handle = page.driver.browser.window_handles.last

Mind these:

  • find_window returns a window handle, which is something like `"{485fa8bd-fa99-…

Git: How to configure git to push only your current branch

You can change which branches will be pushed when saying git push. Our recommendation is to set it to current.

From the git-config documentation:


Defines the action git push should take if no refspec is given on the command line, no refspec is configured in the remote, and no refspec is implied by any of the options given on the command line. Possible values are:

  • nothing - do not push anything.
  • matching - push all matching branches. All bra…

Fix Slack call overlay on Awesome WM

If you use awesome and make a Slack call, you'll constantly have an overlay window pop up taking over half of your screen.

To fix this, add the following rule to your awful.rules.rules section in your rc.lua:

awful.rules.rules = {
  -- ...
  { rule = { name = "Slack Call Minipanel" }, properties = { floating = true, ontop = true } },
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