Writing Code Comments

Code comments allow for adding human readable text right next to the code: notes for other developers, and for your future self.

As always, with great power comes great responsibility. Code comments can go wrong in many ways: they may become outdated, silently move away from the code they're referring to, restate the obvious, or just clutter files.

Good Comments

Here are some simple rules to keep your comments helpful:

Avoid stating what some code does – prefer the Why
Some say, perfect code would not need a single comment. That'…

Carrierwave: Deleting files outside of forms

TL;DR Use the #remove_<mounted_attribute>! method to delete attachments. Also, save the record after removing the file!

As you know, Carrierwave file attachments work by mounting an Uploader class to an attribute of the model. Though the database field holds the file name as string, calling the attribute will always return the uploader, no matter if a file is attached or not. (Side note: use #present? on the uploader to check if the file exists.)

class User < ApplicationRecord
  mount :avatar, AvatarUploader


Whitelist Carrierwave attributes correctly

Say you have a User with a Carrierwave attribute #avatar:

class User < ActiveRecord::Base
  mount_uploader :avatar, AvatarUploader

When whitelisting the avatar field in the controller, you might do this:


But you probably want this:

  params[:user].permit(:avatar, :avatar_cache, :remove_avatar)

In this example:

  • :avatar_cache allows a newly upload image to persist through form roundtrips in the case of validation errors (something that isn't possible with Paper…

Obtain pessimistic row locks on ActiveRecord objects

When requests arrive at the application servers simultaneously, weird things can happen. Sometimes, this can also happen if a user double-clicks on a button, for example.

This often leads to problems, as two object instances are modified in parallel maybe by different code and one of the requests writes the results to the database.

In case you want to make sure that only one of the requests "wins", i.e. one of the requests is fully executed and completed while the other one at least has to wait for the first request to be completed, you ha…


Stop writing "require 'spec_helper'" in every spec

Simply add this to your .rspec instead:

--require spec_helper

If you are on rspec >= 3 and use a rails_helper.rb require this instead of the spec_helper:

--require rails_helper

If you are using parallel_tests and this is not working for you, .rspec might be ignored. Try using a .rspec_parallel file.

Capybara: How to find a hidden field by its label

To find an input with the type hidden, you need to specify the type hidden:

find_field('Some label', type: :hidden)

Otherwise you will see an exception :

find_field('Some label')
# => Capybara::ElementNotFound: Unable to find field "Some label" that is not disabled`.

Note: Usually you don't need to check the input of hidden fields in an integration test. But e.g. waiting for a datepicker library to write the expected value to this field before continuing the test, which prevents flaky tests, is a valid use case.

Quick HTML testing with RubyMine

If you need to test some HTML, e.g. an embed code, you can use RubyMine's "scratch files":

  1. File > New Scratch File (or Ctrl + Shift + Alt + Ins)
  2. Select "HTML" as file type
  3. Write or paste the HTML
  4. Move your mouse to the upper right corner of the scratch file editor. Pick a browser to instantly open your file.

ActiveRecord: Specifying conditions on an associated table

We can use ActiveRecord's where to add conditions to a relation. But sometimes our condition is not on the model itself, but on an associated model. This card explains multiple ways to express this condition using ActiveRecord's query interface (without writing SQL).

As an example we will use a User that has many Posts:

class User < ApplicationRecord
has_many :posts
scope :active, -> { tra…

Letting a DOM element fade into transparency

You can use the CSS property mask-image to define an "alpha channel" for an element.

E.g. to let an element start at full opacity at the top and gradually fade into transparency at the bottom:

.box {
  -webkit-mask-image: linear-gradient(to bottom, black 0%, transparent 100%)
  mask-image: linear-gradient(to bottom, black 0%, transparent 100%)
  • A fully opaque black pixel will render the masked pixel fully opaque
  • A fully transparent black pixel will render the ma…
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Traversing the DOM tree with jQuery

jQuery offers many different methods to move a selection through the DOM tree. These are the most important:

Get the descendants of each element in the current set of matched elements, filtered by a selector. Does not find the current element, even it matches. If you wanted to do that, you need to write $element.find(selector).addBack(selector).
Get the first ancesto…
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Rails: Overriding view templates under certain conditions only

Rails offers a way to prepend (or append) view paths for the current request. This way, you can make the application use different view templates for just that request.


A use case of this is a different set of view templates that should be used under certain circumstances:

class UsersController < ApplicationController

before_action :prepare_views

def index



def prepare_views
if prepend_view_path Rails.root.join('app', 'views', 'special') end end

end …

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Updated: How to make a cucumber test work with multiple browser sessions

  • Simplified step
  • Added section "Effect on other step definitions"
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How to use the Capistrano 2 shell to execute commands on servers

Capistrano 2 brings the shell command which allows you to run commands on your deployment targets.
There is also invoke to run a command directly from your terminal.

Both commands allow running Capistrano tasks or shell commands, and scope to individual machines or machine roles.

Unfortunately Capistrano 3 does not include these commands any more.

cap shell


First of all, spawn a Capistrano shell (we're using the multistage extension here):

$ cap staging shell

In your "cap" shell you can now run Capistrano ta…


How to write modular code

Or: How to avoid and refactor spaghetti code

Please note that I tried to keep the examples small. The effects of the methods in this card are of course much more significant with real / more complex code.

What are the benefits of more modular code?

Code is written once but read often (by your future self and other developers who have to understand it in order to make changes for example). With more modular code you reduce the scope of what has to be understood in order to change something. Also, naming things gives you the opportunity t…

How to make a cucumber test work with multiple browser sessions

Imagine you want to write a cucumber test for a user-to-user chat. To do this, you need the test to work with several browser sessions, logged in as separate users, at the same time.

Luckily, Capybara makes this relatively easy:


Scenario: Alice and Bob can chat
Given Alice, Bob, and a chat session
When I am signed in as "Alice"
And I go to the chat
And I am signed in as "Bob" [session: bob]
And I go to the chat [session: bob]
And I send the message "Hello, this is Alice!"
Then I should see "Hello, this …

Rails: Rest API post-mortem analysis

This is a personal post-mortem analysis of a project that was mainly build to provide a REST API to mobile clients.

For the API backend we used the following components:

  • Active Model Serializer (AMS) to serializer our Active Record models to JSON.
  • JSON Schema to test the responses of our server.
  • SwaggerUI to document the API.

It worked

The concept worked really good. Here are two points that were extraordinary compared to normal Rails project with many UI components:

  • Having a Rails application, that has no UI components (only…

How to: Validate dynamic attributes / JSON in ActiveRecord

PostgreSQL and ActiveRecord have a good support for storing dynamic attributes (hashes) in columns of type JSONB. But sometimes you are missing some kind of validation or lookup possibility (with plain attributes you can use Active Record's built-in validations and have your schema.rb).

One approach about being more strict with dynamic attributes is to use JSON Schema validations. Here is an example, where a project has the dynamic attributes analytic_stats, that we can use to store analytics from an external measurement tool.

  • A g…
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Generating test images on the fly via JavaScript

When you need test images, instead of using services like lorempixel or you may generate test images yourself. You can do this via JavaScript.

Here is an ES6 function that generates a simple SVG image and returns it as a data: URI. All browsers support SVG, and you can easily adjust it yourself.

function svgUri(text) {
  let svg = `
    <svg width="320" height="240" xmlns="">
      <rect x="0" y="0" width="320" height="240" style...
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