Rails: namespacing models with table_name_prefix instead of table_name

When you want to group rails models of a logical context, namespaces are your friend. However, if you have a lot of classes in the same namespace it might be tedious to specify the table name for each class seperately:

class Accounting::Invoice < ApplicationRecord
  self.table_name = 'accounting_invoices'

class Accounting::Payment < ApplicationRecord
  self.table_name = 'accounting_payments'

A replacement for the self.table_name-assignment is the table_name_prefix in the module definition:

module Accou…


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…

Rails: render a template that accepts a block by using the layout option of render

Let's say you have a form that you render a few times but you would like to customize your submit section each time. You can achieve this by rendering your form partial as layout and passing in a block. Your template or partial then serves as the surrounding layout of the block that you pass in. You can then yield back the form to the block and access the form in your block.


= form_for record do |form|

In order to make your template record/_form.haml accept a block whe…

Import Excel files without running into memory limitations

There are several gems that make it easy to read and process xlsx files. Parsing the entire file at once however is error-prone since each cell is transformed to a ruby object - sometimes including thousands of formatted but empty cells.

As of today, I found two promising alternatives that provide a stream-based access to spradsheet rows:

  • Roo supports multiple spreadsheet types like ods or csv and has a quite large contributor base
  • [Creek](…

Dealing with I18n::InvalidPluralizationData errors

When localizing model attributes via I18n you may run into errors like this:

I18n::InvalidPluralizationData: translation data {...} can not be used with :count => 1. key 'one' is missing.

They seem to appear out of the blue and the error message is more confusing than helpful.

The problem

When you run into this, you probably introduced a new model that has attributes like these:

class User < ApplicationRecord
  has_many :posts

class Post < ApplicationRecord
belongs_to :user
validates :user, presence: true …


Ruby: A small summary of what return, break and next means for blocks


  • Use return to return from a method. return accepts a value that will be the return value of the method call.
  • Use break to quit from a block and from the method that yielded to the block. break accepts a value that supplies the result of the expression it is “breaking” out of.
  • Use next to skip the rest of the current iteration. next accepts an argument that will be the result of that block iteration.

The following method will serve as an example in the details below:

def example(&block)


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) { }
  it { ... }

# TestRecord will exist here, outside of the spec!

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


How Ruby method lookup works

When you call a method on an object, Ruby looks for the implementation of that method. It looks in the following places and uses the first implementation it finds:

  1. Methods from the object's singleton class (an unnamed class that only exists for that object)
  2. Methods from prepended modules (Ruby 2.0+ feature)
  3. Methods from the object's class
  4. Methods from included modules
  5. Methods from the class hierarchy (superclass and its ancestors)


Let's say we h…

How to change the class in FactoryGirl traits

FactoryGirl allows a :class option to its factory definitions, to set the class to construct. However, this option is not supported for traits.

Most often, you can just define a nested factory instead of a trait, and use the :class option there.

factory :message do
  factory :reply, class: Message::Reply do
    # ...

If you need/want to use traits instead (for example, it might make more sense semantically), you can not use a :class on a trait.

In that case, use initialize_with to define the record's const…

Ubuntu MATE: Fixing ALT + TAB being really slow

On my Ubuntu MATE machine, switching applications with ALT + TAB was impossible, because it took nearly 2 seconds. The culprit appears to be an "UX improvement" gone bad: MATE renders a live screenshot of each running application inside the application switcher.

Fixing this is simple:

  • Open "Window Preferences"
  • Either uncheck "Enable software compositing window manager" or check "Disable thumbnails in Alt-Tab"

In MySQL, a zero number equals any string

In MySQL comparing zero to a string 0 = "any string" is always true!

So when you want to compare a string with a value of an integer column, you have to cast your integer value into a string like follows:

SELECT * from posts WHERE CAST(posts.comments_count AS CHAR) = '200' 

Of course this is usually not what you want to use for selecting your data as this might cause some expensive database operations. No indexes can be used and a full table scan will always be triggered.

If possible, cast the compared value in your application to…

Postgres: How to force database sessions to terminate

If another session is accessing your database you are trying to reset or drop you might have seen the following error:

PG::ObjectInUse: ERROR:  database "foo_development" is being accessed by other users
DETAIL:  There is 1 other session using the database.

This could be the rails server, rubymine and many more. Beside terminating the session connection manually you can also find out the pid and kill the process.

1. rails db
2. SELECT * FROM pg_stat_activity;

datid | 98359
datname | foo_developm…


Project management best practices: Standup

If the project team consists of at least 2 members, do a daily standup. It should not take much longer than 15 minutes.


Tell everyone else

  • what you did yesterday
  • what you intend to do today
  • where you might need help or other input
  • if there are new developments everyone needs to know about

A "still working on X, will probably be done today" is totally fine. No need to tell a long story.

If you are out of work, find a new story with the others.

If there a new stories in the backlog, look at them and

  • make sure ever…

How to enable the Thinkpad microphone mute key on Ubuntu 16.04

While the hardware mute button of my Lenovo x230 worked on Ubuntu 14.04 out of the box, it does not on Ubuntu 16.04. It is fairly simple to fix, though.

There is an extensive answer on Ask Ubuntu, but only part of it was required for me. Here is the gist of it.

  1. Open a terminal

  2. Run acpi_listen and press the mute key. You should see something like this:

    button/f20 F20 00000080 00000000 K

    Press Ctrl+C to exit.

  3. Run amixer scontrols. You will see multiple lines, one of which sh…


How to avoid ActiveRecord::EnvironmentMismatchError on "rails db:drop"

After loading a staging dump into development, you might get an ActiveRecord::EnvironmentMismatchError when trying to replace the database (like rails db:drop, rails db:schema:load).

$ rails db:drop
rails aborted!
ActiveRecord::EnvironmentMismatchError: You are attempting to modify a database that was last run in `staging` environment.
You are running in `development` environment. If you are sure you want to continue, first set the environment using:

        bin/rails db:environment:set RAILS_ENV=development

Starting with R…

Sentry: How to get a list of events that is programmatically processable

The linked documentation explains how to get a JSON list of the latest 100 events of a given issue. Simply open this URL while logged in:<issue id>/events/

How to send HTTP requests using cURL

  • Reading a URL via GET:

  • Defining any HTTP method (like POST or PUT):

    curl -XPUT
  • Sending data with a request:

    curl -d"first_name=Bruce&last_name=Wayne"

    If you use -d and do not set an HTTP request method it automatically defaults to POST.

  • Performing basic authentication:

  • All together now:

    curl -XPUT -d"screen_name=batman"

How to pair a Bose Quiet Comfort 35 with your Ubuntu computer

You need to disable "Bluetooth low energy", then follow these core steps:

1) Make sure the headphones are in pairing mode.
2) Pair with System Settings > Bluetooth
3) Select & test the headphones in System Settings > Sound. Choose High Fidelity Playback (A2DP Sink).

I also had to install a package with sudo apt-get install pulseaudio-module-bluetooth and load it with pactl load-module module-bluetooth-discover. Put the latter command into ~/.bashrc or you'll need to run it after each boot.


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