3577 cards

Take care of indentation and blank lines when using .erb for plain text emails

Building plain text emails with an .erb template doesn't allow you to indent code like you normally do in HTML mails.


<%= 'foo' if bar %>

"\n" if bar is false

"foo\n" if bar is true

<%= nil %>


<% if true %>
  <%= 'foo' %>
<% end %>  

" foo"

<%= 'foo' %>

<%= 'bar' %>



Write unindented code to get the expected result.

<% if bar %>
<%= 'bar' %>
<% end %>
<%= 'foo' %>
<%= 'bar' %> 
  • Use [Form Models](https://github.com/makandra…

Test if two date ranges overlap in Ruby or Rails

A check if two date or time ranges A and B overlap needs to cover a lot of cases:

1. A partially overlaps B

2. A surrounds B

3. B surrounds A

4. A occurs entirely after B

5. B occurs entirely after A

This means you actually have to check that:

  • neither does A occur entirely after B (meaning A.start > B.end)
  • nor does B occur entirely after A (meaning B.start > A.end)

Flipping this, A and B overlap iff A.start <= B.end && B.start <= A.end

The code below shows how to implement this in Ruby on Rails. The example is a clas…


Your Rails sandbox console

Just found out about a great feature in Rails that seems to be around since Rails 2. Start a console with the --sandbox (or -s) parameter:

rails console --sandbox

All changes you make to the database will be rolled back on exit.

Note however, that changes beyond the database (deleting files, sending emails, etc) cannot be rolled back!

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How to let passenger restart after deployment with capistrano

Phusion Passenger changed the way how it gets restarted several times. Through the project's history, these all were valid:

  • touch tmp/restart.txt
  • sudo passenger-config restart-app /path/to/app
  • passenger-config restart-app /path/to/app

You should not need to know which one to use. Instead, the capistrano-passenger gem will choose the appropriate restart mechanism automatically based on your installed the passenger version.


  1. Add to your Gemfile:

    gem 'capistr...
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Drawing diagrams with draw.io

draw.io is a web application that lets you draw flow charts and other diagrams. It works much nicer than office suite tools like LibreOffice Draw.

Its UI is easy to use: Simply drag and drop shapes or arrows from the left-hand side onto your sheet. Drag start/end of arrows to shapes to automatically connect them. You can add text to shapes by selecting them and typing. To add colors, select a shape and use the right-hand panel.

While you can, you don't need to store your charts online. "Offline" storage can be files…

RSpec: How to check if a string contains terms in a desired order

There seems to be no built-in matcher in RSpec to check if a string contains terms in the desired order. A simple workaround is to use a regular expression that also matches newlines (m-modifier).


  • The readability when terms need to be escaped is bad
  • A failed spec has an error which needs manually action to be understood (Search if terms appear and in which order)


expect(ActionMailer::Base.last.to_s).to match(/Dear customer.*Account canceled.*You Awesome Company/m)

When defining methods in specs, you can super into previous definitions

When you def methods in RSpec tests, you can use super to call a method definition that was created in a surrounding describe/context block.


describe User do

  def build_user(attributes = {})

  describe '#my_method' do
    it 'does something' do
      # ...

    context 'when the user has an e-mail address' do
      def build_user(attributes = {})
        super({ email: 'user@example.com' }.merge(attributes))

      it 'd...

An incomplete guide to migrate a Rails application from paperclip to carrierwave

In this example we assume that not only the storage gem changes but also the file structure on disc.

A general approach

Part A: Create a commit which includes a script that allows you to copy the existing file to the new file structure.

Part B: Create a commit which removes all paperclip logic and replace it with the same code you used in the first commit

Part A

Here are some implementation details you might want to reuse:

  • Use the existing models to read the files from
  • Use your own carrierwave models to write t…

Debugging cucumber feature with javascript + firefox vnc

TL;DR Debugging problems with javascript errors in cucumber tests is sometimes easier in the browser. Run the test, stop at the problematic point (with Then pause from Spreewald 1.7+) and open VNC for Firefox.



Everything you know about html_safe is wrong

Back in the war, Rails developers had to manually HTML-escape user-supplied text before it was rendered in a view. If only a single piece of user-supplied text was rendered without prior escaping, it enabled XSS attacks like injecting a <script> tag into the view of another user.

Because this practice was so error-prone, the rails_xss plugin was developed and later integrated into Rails 3. rails_xss follows a different approach: Instead of relying…


Cancelling the ActiveRecord callback chain

What Rails version Within before_* Within after_*
Cancel later callbacks Rails 1-4 return false return false
Cancel later callbacks Rails 5 throw :abort throw :abort
Rollback the transaction Rails 1-4 return false raise ActiveRecord::Rollback
Rollback the transaction Rails 5 `throw…  
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Colcade is a lightweight Masonry alternative

Masonry is a famous library to dynamically arrange a grid of items that have different aspect ratio, like horizontal and vertical images. Colcade is an alternative masonry-layouting library, developed by the same developer, but with a more modern approach.

It is said to have better performance while being smaller and having no dependencies. It automagically detects jQuery and defines a jQuery initializer, if present.
However, it offers [a few less features](https:…


A quick introduction to CORS


Cross-Site Request Forgery (CSRF) is an attack pattern for websites. A CSRF attack is usually relevant in a browser context, where state is kept for multiple domains (as opposed to independent requests made e.g. with curl). The most common example is authentication via cookies. If a script on https://example.com made requests to https://docs.google.com, the browser would send all cookies for docs.google.com along, effectively given the script access to anythin…

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Heads up: Sidekiq per default silently fails when retries are exhausted!

For Sidekiq to be able to retry your jobs it has to be able to catch errors that occur while a job is executed.

Per default, Sidekiq will not raise / notify you if the retry count is exhausted. It will only copy the job to the dead queue (see wiki).

If you want to get notified, you have to implement it in your worker explicitly with a sidekiq_retries_exhausted-block, e.g. like this:

class DownloadWorker
include Sidekiq::Worker

# Import jobs are retried a few times wi…


Javascript: Calling a function with a variable number of arguments

This card describes how to pass an array with multiple element to a JavaScript function, so that the first array element becomes the first function argument, the second element becomes the second argument, etc.

Note how this is different from passing the entire array as the first argument. Compare these two different ways of calling fun() in Ruby:

# Ruby
array = [1, 2, 3]
fun(array)  # same as fun([1, 2, 3]) (1 argument)
fun(*array) # same as fun(1, 2, 3)   (3 arguments)

Depending on your culture the spreading of array e…

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InfoQ: How to Design a Good API & Why it Matters

A well-written API can be a great asset to the organization that wrote it and to all that use it. Given the importance of good API design, surprisingly little has been written on the subject. In this talk (recorded at Javapolis), Java library designer Joshua Bloch teaches how to design good APIs, with many examples of what good and bad APIs look like.


Case sensitivity in PostgreSQL

PostgreSQL, unlike MySQL, treats strings as case sensitive in all circumstances.

This includes

  • comparison with = and LIKE
  • collision detection in unique indexes

Usually this is fine, but some strings (like emails and usernames) should typically be treated as case insensitive.

There are a few workarounds available:

  • use the citext extension
  • use ILIKE instead of LIKE
  • use Postgres' lower() function
  • add an index on lower(email)

Probably the best approach is…


Raising JavaScript errors in Cucumber scenarios and how to silence them for particular scenarios

Raising JavaScript errors in Cucumber

With selenium JavaScript errors do not cause any failture in your cucumber scenarios. To archive a failture we can use the the Ruby bindings to Selenium Webdriver expose the Webdriver logging API introduced in version 2.38 .

The following step can be used:

Then /^there should be no JavaScript errors$/ do
if alert_present?
# Chrome 54 and/or Chromedriver 2.24 introduced a breaking change on how
# accessing browser logs work.
# Apparently, while an alert/confirm is open, Chro…

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