3674 cards

Update a projects bundler version

We use bundler to maintain a projects gems. Unfortunately bundler v1 itself is incompatible with bundler v2.
You have two ways to update bundler on the server.

If you use our gem capistrano-opscomplete in your project, you can tell opscomplete to install the required bundler version:

Add the following line to deploy.rb. If you use different deployment targets, add the follwoing line to the specific target configuration file. (e.g `product…


Thread-safe collections in Ruby

When using threads, you must make your code thread-safe. This can be done by either locking (mutexes) all data shared between threads, or by only using immutable data structures. Ruby core classes like String or Array are not immutable.

There's several gems providing thread-safe collection classes in Ruby.


The concurrent-ruby gem provides thread-safe versions of Array and Hash:

sa = Concurrent::Array.new # supports standard Array.new forms
sh = Conc…


RSpec: How to define classes for specs

RSpec allows defining methods inside describe/context blocks which will only exist inside them.
However, classes (or any constants, for that matter) will not be affected by this. If you define them in your specs, they will exist globally. This is because of how RSpec works (short story: instance_eval).

describe Notifier do
  class TestRecord
    # ...
  let(:record) { TestRecord.new }
  it { ... }

# TestRecord will exist here, outside of the spec!

This will come bite you at least when you try to define a clas…

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How to: Use git bisect to find bugs and regressions

Git allows you to do a binary search across commits to hunt down the commit that introduced a bug.

Given you are currently on your branch's HEAD that is not working as expected, an example workflow could be:

git bisect start # Start bisecting
git bisect bad # Tag the revision you are currently on (HEAD) as bad. You could also pass a commit's SHA1 like below:
git bisect good abcdef12345678 # Give the SHA1 of any commit that was working as it should
# shorthand:
git bisect start <bad ref> <good ref>

Git will fetch a comm…

Joining PDFs with Linux command line

There are several ways to merge two (or more) PDF files to a single file using the Linux command line.

If you're looking for graphical tools to edit or annotate a PDF, we have a separate card for that.


The pdfjoin binary from pdfjam is probably already installed on your system. Usage:

pdfjoin one.pdf two.pdf --outfile out.pdf

Pages from joined documents might be rotated. To avoid this, call it like this:

pdfjoin one.pdf two.pdf --outfile out.pdf --paper a4paper --rotateoversize false



Hiding the clear input button of Edge (with EdgeHTML engine)

Edge (and some versions of Internet Explorer, like IE11) use to render a × clear input button on text fields. While this is intended as a nicety for users, it comes in annoying when you've built and styled your own clear input button.

Hide the Edge × with this pseudo selector:

  display: none
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IE11 ignores calc() functions in the flex shorthand

Example (broken in IE):

flex: 0 1 calc(50% - 20px)


flex-basis: calc(50% - 20px)

flex-grow: 0 // Default, can be omitted for this example
flex-shrink: 1 // Default, can be omitted for this example
Linked content

The Difference Between Width and Flex Basis | Geddski

Within a Flexbox layout, there are multiple CSS attributes that may affect a child's basis (the initial width before flexing). You might be confused how flex-basis, width, min-width and the intrinsic width of your content play together.

The attached article explains the differences. In summary:

  • If a flex-basis is set, that is used as the basis
  • If no flex-basis is set, the width is used as the basis
  • If neither flex-basis nor width is set, the content…
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Faking and testing the network with WebMock

An alternative to this technique is using VCR. VCR allows you to record and replay real HTTP responses, saving you the effort to stub out request/response cycles in close details. If your tests do require close inspection of requests and responses, Webmock is still the way.

WebMock is an alternative to FakeWeb when testing code that uses the network. You sh…

Class methods in Coffeescript

How to define and call class methods in CoffeeScript classes:

class Foo

  @classMethod: -> 
  instanceMethod: ->

Google Analytics: Change the tracked URL path

By default, Google Analytics tracks the current URL for every request. Sometimes you will want to track another URL instead, for example:

  • When an application URL contains a secret (e.g. an access token)
  • When you want to track multiple URLs under the same bucket
  • When you want to track interactions that don't have a corresponding URL + request (e.g. a Javascript button or a PDF download)

Luckily the Analytics code snippet allows you to freely choose what path is being tracked. Simple change this:

ga('send', 'pageview');

… to th…


Rails: Do not load frameworks you don't need

Rails is split into a large number of (sub-) frameworks.

The most important and central of those are

  • activesupport (extends the Ruby standard library)
  • activerecord / activemodel (ORM for Rails)
  • actionview / actionpack (controller / views)
  • actionmailer (sends mails)

However, there are also some more situational frameworks included, such as

  • actioncable (real time communications using websockets)
  • actionmailbox (receives mails)
  • actiontext (support for WYSIWYG text editor)
  • activejob (background jobs)
  • activestorage (file uplo…

Take care of existing users when upgrading Clearance

When upgrading Clearance, pay attention whether the password hashing strategy might have changed. Old clearance versions (< 1.0) used SHA1-encrypted passwords by default. Current versions default to BCrypt.

If you simply upgrade without taking this into account, users will get a BCrypt::Errors::InvalidHash when trying to sign in. Your tests will not notice this, since they create new users for each scenario.

To fix it, you'll either have to force all users to reset their passwords, or you can allow old users to keep signing in with t…

Known MariaDB version incompatibilities

MariaDB (and MySQL) is released in different versions with different behaviors. For backwards compatibility this can be managed with the sql_mode option.

Following you can find behavior differences which cannot be changed by this option. This can cause a regression in an existing application. There is no workaround available. The application needs to be adapted.

Queries with aggregated date types

For a database instance of MariaDB-10…

Fix for "Rails assets manifest file not found" in Capistrano deploy

If you use webpacker in your Rails application, and you have completely disabled Sprockets, you might get the following error when trying to deploy: Rails assets manifest file not found. This happens inside the deploy:assets:backup_manifest task.

This task is build for Sprockets and does not work with webpacker. Simply skip it, by adding the following line to your config/deploy.rb:


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"    ...
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Casting ActiveRecord scopes or instances to ActiveType extended model classes

When working with ActiveType you will often find it useful to cast an ActiveRecord instance to its extended ActiveType::Record variant.

Starting with active_type 0.4.0 you can use ActiveType.cast for this:

class User < ActiveRecord::Base

class SignUp < ActiveType::Record[User]

user = User.find(1)
sign_up = ActiveType.cast(user, SignUp)
sign_up.is_a?(SignUp) # => true

This is basically like [ActiveRecord#becomes](http://apidock.com/rails/v4.2.1/ActiveRecord/Pers…

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Linux: Running a single unique instance of command + arguments

run-one is a wrapper script that won't run the given command while another instance of it is running. Is brings several utility commands that offer similar behavior.

run-one - run just one instance at a time of some command and unique set of arguments (useful for cronjobs, eg)

run-one COMMAND [ARGS]

   run-this-one COMMAND [ARGS]

   run-one-constantly COMMAND [ARGS]

   keep-one-running COMMAND [ARGS]

   run-one-until-success COMMAND [ARGS]

   run-one-until-failure COMMAND [ARGS]

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