CSS: :is() pseudo selector


The :is() pseudo selector - specificity of its most specific argument - matches against a comma-separated list of selectors.


Compound selectors like ...

.datepicker .prev, .datepicker .next, .datepicker .switch
  padding-bottom: 1rem

ul li, ol li
  list-style-type: none

can be simplified by using the :is() pseudo selector ...

.datepicker :is(.prev, .next, .switch)
  padding-bottom: 1rem

:is(ul, ol) li
  list-style-type: none


The specificity of :is() is equals t...

CSS: :where() pseudo selector


The :where() pseudo selector - zero specificity - matches against a comma-separated list of selectors.


Compound selectors like ...

.datepicker .prev, .datepicker .next, .datepicker .switch
  padding-bottom: 1rem

ul li, ol li
  list-style-type: none

can be simplified by using the :where() pseudo selector ...

.datepicker :where(.prev, .next, .switch)
  padding-bottom: 1rem

:where(ul, ol) li
  list-style-type: none


The specificity of :where() is always zero!


Don't sum up columns with + in a SQL query if NULL-values can be present.

Don't sum up columns with + in a sql-query if NULL-Values can be present.

MySQL and PostgreSQL cannot sum up NULL values with the + value. The sum value will be NULL.


mysql> select 1 + 2 + 3;
| 1 + 2 + 3 |
|         6 |
1 row in set (0,00 sec)

mysql> select 1 + NULL + 3;
| 1 + NULL + 3 |
|         NULL |
1 row in set (0,00 sec)


test_database=# select 1 + 2 + 3;

(1 row)


Rails and Postgres: How to test if your index is used as expected

This is a small example on how you can check if your Postgres index can be used by a specific query in you Rails application. For more complex execution plans it might still be a good idea to use the same path of proof.

1. Identify the query your application produces

query = User.order(:last_name, :created_at).to_sql
puts query
# => SELECT "users".* FROM "users" ORDER BY "users"."last_name" ASC, "users"."created_at" ASC

2. Add an index in your migration and migrate

add_index :users, [:last_name, :created_at]

How to use Ubuntu in English, but still show German formats

If you want to have an English Ubuntu UI, but still see dates, money amounts, paper formats, etc. in German formats, you can fine-tune your /etc/default/locale like this:


Make sure you have both en...

Rails: How to check if a certain validation failed

If validations failed for a record, and you want to find out if a specific validation failed, you can leverage ActiveModel's error objects.
You rarely need this in application code (you usually just want to print error messages), but it can be useful when writing tests.

As an example, consider the following model which uses two validations on the email attribute.

class User < ApplicationRecord
  validates :email, presence: true, uniqueness: true

Accessing errors

Let's assume we have a blank user:

user = Us...

Generating and streaming ZIP archives on the fly

When your Rails application offers downloading a bunch of files as ZIP archive, you basically have two options:

  1. Write a ZIP file to disk and send it as a download to the user.
  2. Generate a ZIP archive on the fly while streaming it in chunks to the user.

This card is about option 2, and it is actually fairly easy to set up.

We are using this to generate ZIP archives with lots of files (500k+) on the fly, and it works like a charm.

Why stream downloads?

Offering downloads of large archives can be cumbersome:

  • It takes time to b...

Rails: Fixing the memory leak / performance issues in prepend_view_path

Recently we detected a memory leak in one of our applications. Hunting it down, we found that the memory leak was located in Rails' #prepend_view_path. It occurs when the instance method prepend_view_path is called in each request, which is a common thing in a multi-tenant application.

On top of leaking memory, it also causes a performance hit, since templates rendered using the prepended view path will not be cached and compiled anew on each requqest.

This is not a new memory leak. It was [first reported in in 2014](https://github.com...

Showing a custom maintenance page while deploying

Add a custom maintenance page for each vhost. We have automated this with capistrano 3.x, but you can do it manually by following the instructions at the bottom:


Add this line to your application's Gemfile:

gem 'capistrano', '~> 3.0'
gem 'capistrano-maintenance', '~> 1.0'

Add this line to you application's Capfile:

require 'capistrano/maintenance'

Enable task

Present a maintenance page to visitors. Disables your application's web interface by writing a #{maintenance_basename}.html file to each web ...

Cucumber pitfall: "Around" does not apply to your "Background" steps

Around will not happen until after a feature's Background has been processed. Use Before and After to avoid that.


Consider this Cucumber feature file:

Feature: Something that needs to be tested

    Given a user
      And I sign in        
  Scenario: Sign out
    When I sign out
    Then I should see "Signed out"
  Scenario: Something else
    # ...

Now, assume you have these step definitions:

Around do
  puts "** Around: before yield"


Valuable Chrome DevTools Shortcuts

In the DevTools settings, there's a "Shortcuts" section. Found these keyboard shortcuts there:



Toggle drawer

CTRL + ~ or CTRL + `

Show console in drawer


SHIFT + up/down

Change number by 10

CTRL + up/down

Change number by 100



Toggle "visibility:hidden!important" (useful when debugging page repaint times)

CTRL + hover above element in the DOM list

Don't show the yellow dimensions tooltip (useful when the tooltip covers just the area you need to see).


A short overview of common design patterns implemented within Rails

The linked content includes a few design patterns implemented with Ruby on Rails.

What is the card indented to achieve?

  • You can use the pattern names for code reviews, so all parties know with only a few words which change is requested. Example: "Please use a form object here"
  • You can learn about new code patterns
  • You should read the sections "Advantages of using design patterns" and "Disadvantages of using design patterns in a wrong way", since design patterns do not replace good code

Included Design Patterns: Service, Value objec...

makandra tech survey - results

These are the results of the "personal tech stack survey". I've included only the most popular mentions, maybe it can help you find one or two useful tools for your own usage.

Desktop environment

pie title Desktop environment
    "Gnome" : 16
    "i3": 2
    "sway": 2
    "awesome": 1
    "bspwm": 1
    "mate": 1
    "xfce": 1

Gnome dominates (unsuprising, it's the Ubuntu default), but quite a few people use tiling window managers, most popular i3 Archive and the mostly i3-compatible [sway](https://swaywm....

Fix for mysql2 error "Incorrect MySQL client library version! This gem was compiled for x.x.x but the client library is y.y.y."

This should be fixed in the latest LTS-branches of our mysql2 fork Archive , 0.2.x-lts and 0.3.x-lts.


gem 'mysql2', git: 'https://github.com/makandra/mysql2', branch: '0.2.x-lts' # for Rails 2.x
gem 'mysql2', git: 'https://github.com/makandra/mysql2', branch: '0.3.x-lts' # for Rails 3.x

in your Gemfile, and do a

bundle update mysql2


mysql2 used to check that the client library used at runtime actually matches the one it was compiled against. However, at least on Ubunt...

Reading the Rails session hash from a Rack middleware

To read the Rails session from a Rack middleware, use env['rack.session']. It's an ActionDispatch::Request::Session object.

class MyMiddlware

  def initialize(app)
    @app = app

  def call(env)
    status, headers, body = @app.call(env)
    session = env['rack.session']
    Rails.logger.info("Value of session['foo'] is: " + session['foo'].inspect)
    [status, headers, body]


You may not be able to write to the session this way (I haven't tested this).

Fixing flaky E2E tests

An end-to-end test (E2E test) is a script that remote-controls a web browser with tools like Selenium WebDriver. This card shows basic techniques for fixing a flaky E2E test suite that sometimes passes and sometimes fails.

Although many examples in this card use Ruby, Cucumber and Selenium, the techniques are applicable to all languages and testing tools.

Why tests are flaky

Your tests probably look like this:

When I click on A
And I click on B
And I click on C
Then I should see effects of C

A test like this works fine...

Use a global .gitignore file to ignore stuff from your machine

Sometimes you want git to ignore certain files that appear on your machine. You can do this in 3 ways:

  • Per project, in the project's .gitignore file
  • Per project, in a local exclude file
  • Globally on your machine

Downsides of per-project .gitignore entries

While it might be tempting to set it per project (other devs might benefit from it), you

  • need to do it each time for every project
  • "pollute" a project's .gitignore file with stuff...

Running "bundle update" without arguments might break your application

Calling bundle update (without arguments) updates all your gems at once. Given that many gems don't care about stable APIs, this might break your application in a million ways.

To stay sane, update your gems using the applicable way below:

Projects in active development

Update the entire bundle regularily (e.g. once a week). This ensures that your libraries are up-to-date while it's easy to spot major version bumps which may break the app.

Projects that have not been updated in a while

  1. [Update a single gem conservatively](htt...