Rails 7.1: Take care of the new production log default to standard out

Starting with Rails 7.1 the production logger is set to standard out. For applications running with opscomplete ensure to keep the file logging.

It should be enough to change these lines in the config/environments/production.rb back to the implementation in Rails <7.1:

-  # Log to STDOUT by default
-  config.logger = ActiveSupport::Logger.new(STDOUT)
-    .tap  { |logger| logger.formatter = ::Logger::Formatter.new }
-    ...

Problems with git submodules in Gitlab CI

If you are using git submodules in Gitlab CI, you might run into a "The project you were looking for could not be found or you don't have permission to view it."

Gitlab added a feature that new projects are no longer allowed to be cloned inside CI runs of other repositories by default. To fix this

  • Go into the project used as a submodule
  • Go to "Settings" -> "CI/CD" (if you don't see this section, enable it in "Settings" -> "General" -> "Visibility, project features, permissions")
  • Go to "Token Access"
  • Either disable "Limit access to ...

Cucumber's table diffing does not play nice with Spreewald's `patiently do`

Turns out, Cucumber::MultilineArgument::DataTable#diff! caches some stuff. Code of the following form will not work as intended:

Then('some table should look like') do |expected_table|
  patiently do
    actual_table = calculate_actual_table
    expected_table.diff!(actual_table) # not actually patient, will keep failing if it failed the first time

Instead, simply use


Debug SAML in development using a local keycloak server

Developing or debugging SAML functionality can be a hassle, especially when you need to go back and forth with someone external who is managing the identity provider (IDP).
But you can setup a local keycloak server to act as your IDP to play around with. This might seam intimidating, but is actually quite simple when using docker and turning off some verification steps.

1. Start a keycloak instance using docker

`mkdir -p keycloak_data && docker run -p 8080:8080 -e KEYCLOAK_ADMIN=admin -e KEYCLOAK_ADMIN_PA...

Ruby: How to connect to a host with expired SSL certificate

If you need to make an HTTPS connection to a host which uses an expired certificate, do not disable certificate verifications entirely. Doing that enables e.g. man in the middle attacks.
If you accept only a single expired and known certificate, you are much less in trouble.


All the solutions described below use a verify_callback for the request's OpenSSL::X509::Store where you can specify a lambda to adjust its verification response.
Your callback must return either true or false and OpenSSL's verification result is...

Your Cronjobs should not rely on a perfect schedule

Due to network or hardware failures, it can happen that one of your cronjobs will not run at the time you specify in the schedule. Your code should be built in a way that it can be re-run at a later time (when the failure is resolved).

For example, if you are synchronizing data with another service once every day, your cronjob should not only synchronize changes from the last 24 hours. If you do this and a network failure will delay the execution of your job by 5 hours, you will only synchronize changes from hour 6-29, but forget change...

Stretching an HTML page to full height

This card existed before, but was outdated due to browser implementation changes. The information below is validated for the current list of browsers we support.

By default your html and body elements are only as high as the actual page content inside their container. If you only have two lines of text in your page, your html and body elements will only be around 40 pixels high, regardless of the size of your browser window.

You might be surprised by this, since setting a background on either html and `body...

Triggering JavaScript when an element is clicked

Often people need links which are not linked directly, but should trigger execution of JavaScript.

❌ Bad workarounds

You can find a lot of workarounds for that:

  • <a href="#">Do something with js!</a>
    This defines an empty anchor. This may lead the browser to let the page jump to the top when the link is clicked, unless you call preventDefault on the event. This is probably not what you want.

  • <a href="#!">Do something with js!</a>
    This tells the browser to jump to an anchor !. It depends on the browser implementation wha...

Auto-generating plain-text bodies for HTML e-mails in Rails apps

When building an application that sends e-mails to users, you want to avoid those e-mails from being classified as spam. Most obvious scoring issues will not be relevant to you because you are not a spammer.

However, your application must do one thing by itself: When sending HTML e-mails, you should include a plain-text body or tools like SpamAssassin will apply a significant score penalty. Here is how to do that automatically.

  1. Add premailer-rails to your Gemfile and bundle.
  2. Done! ...

DNS debug tools

There are several tools for DNS debugging which offer you more or less information. Most of the time the more simple ones, like host oder nslookup will be sufficient.


simple DNS lookup utility.

>host heise.de
heise.de has address
heise.de has IPv6 address 2a02:2e0:3fe:1001:302::
heise.de mail is handled by 10 relay.heise.de.


query Internet domain name servers. Nslookup has two modes: interactive and non-interactive.

>nslookup heise.de

RSpec matcher to compare two HTML fragments

The RSpec matcher tests if two HTML fragments are equivalent. Equivalency means:

  • Whitespace is ignored
  • Types of attribute quotes are irrelevant
  • Attribute order is irrelevant
  • Comments are ignored

You use it like this:

html = ...
expect(html).to match_html(<<~HTML)
    Expected content

You may override options from CompareXML by passing keyword arguments after the HTML string:

html = ...
expect(html).to match_html(<<~HTML, ignore_text_nodes: true)

Why has_many :through associations can return the same record multiple times

An association defined with has_many :through will return the same record multiple times if multiple join models for the same record exist (a n:m relation). To prevent this, you need to add ->{ uniq } as second argument to has_many (below Rails 4 it is a simple option: has_many :xyz, :uniq => true).


Say you have an Invoice with multiple Items. Each Item has a Product:

class Invoice < ActiveRecord::Base
  has_many :items
  has_many :products, :through => :items

class Item < ActiveRecord::Base

Git: Switching back to the previous branch

Using git checkout - you can switch back to the branch you previously worked on.

(master) $ git checkout foobar
Switched to branch 'foobar'
(foobar) $ git checkout -
Switched to branch 'master'
(master) $

This also works with other commands like git merge:

(master) $ git checkout foobar
Switched to branch 'foobar'
(foobar) $ git merge -
Merged branch 'master'

Rails: Testing file downloads with request specs


Prefer request specs over end-to-end tests (Capybara) to joyfully test file downloads!


Testing file downloads via Capybara is not easy and results in slow and fragile tests. We tried different approaches and the best one is just okay.

Tests for file downloads via Capybara ...

  • ... are slow,
  • ... are fragile (breaks CI, breaks if Selenium driver changes, ...),
  • ... need workarounds for your specia...

List of handy Ruby scripts to transcode different file types (often by using GPT)

It's 2024 and we have tools like ffmpeg, imagemagick and GPT readily available. With them, it's easy to convert texts, images, audio and video clips into each other.

For the everyday use without any parameter tweaking I'm using a collection of tiny scripts in my ~/bin folder that can then be used as bash functions. And: It's faster to use the CLI than interacting with a website and cheaper to use the API than buying GPT plus.. :-)


  • text-to-image "parmiggiano cheese wedding cake, digital art"
  • `text-to-audio "Yesterday I ate ...

MySQL: Disable query cache for database profiling

If you want to see how long your database queries actually take, you need to disable MySQL's query cache. This can be done globally by logging into a database console, run

SET GLOBAL query_cache_type=OFF;

and restart your rails server.

You can also disable the cache on a per query basis by saying


You also probably want to disable Rails internal (per-request) cache. For this, wrap your code with a call to ActiveRecord::Base.uncached. For example, as an around_filter:


Carrierwave processing facts

  1. Class-level process definitions are only applied to the original file
  2. Versions are generated based on the processed original file
  3. Callbacks (before/after) are applied to original file and each version by itself
  4. Under the hood, a version is an instance of the uploader class that has no versions
  5. Version uploader and original uploader can be distinguished by checking #version_name: version uploaders return the version name, whereas the original uploader instance returns nil
  6. Version instances do not have a re...