At makandra, we've built a few gems over the years. Some of these are quite popular: spreewald (475k downloads), active_type (330k downloads), and geordi (210k downloads) for example (numbers from 2018).

Developing a Ruby gem is different from developing Rails applications, with the biggest difference: there is no Rails. This means:

• no defined structure (neither for code nor directories)
• no autoloading of classes, i.e. you need to require all files yourself
• no [active_support](https://api.rubyonrails.org/classes/ActiveSupport.ht…

If you need to run a program on a remote machine (e.g. to your office PC) with a graphical UI (and you trust the remote machine), you can use SSH X-Forwarding. I sometimes use this to connect to a virtual machine installed on my work PC from my home office.

Forwarding X over SSH

To use X forwarding, when connecting to the remote machine, and add -X to the ssh call. Now, when you start a program with a UI (e.g. virtualbox) in that SSH session, a window will open on your local machine. It will not be particularly …

Migrating data from a legacy into a new system can be a surprisingly large undertaking. We have done this a few times. While there are significant differences from project to project, we do have a list of general suggestions.

Before you start, talk to someone who has done it before, and read the following hints:

Understand the old system

Before any technical considerations, you need to understand the old system as best as possible. If feasible, do not only look at its API, or database, or frontend, but let a user of the old system sho…

PostgreSQL uses the C library's locale facilities for sorting strings:

• First, all the letters are compared, ignoring spaces and punctuation.
• Then, spaces and punctuation are compared to break ties.
• It sorts upper and lower case letters together. So the order will be something like a A b B c C

Example:

[PostgreSQL-FAQ: Why_do_my_strings_sort_incorrectly](https://wiki.postgresql.org/wiki…

In the following example the method update_offices_people_count won't be called when office_id changes, because it gets overwritten by the second line:

after_save :update_offices_people_count, :if => :office_id_changed? # is overwritten …
after_save :update_offices_people_count, :if => :trashed_changed? # … by this line

Instead write:

after_save :update_offices_people_count, :if => :office_people_count_needs_update?

private

def office_people_count_needs_update?
  office_id_changed? || trashed_changed?
end

Or, move t…

Having a unique selector for an element is useful to later select it from JavaScript or to update a fragment with an Unpoly.

Haml lets you use square brackets ([]) to generate a unique class name and ID from a given Ruby object. Haml will infer a class attribute from the given object's Ruby class. It will also infer an id attribute from the given object's Ruby class and #id method.

This is especially useful with ActiveRecord instances, which have a persisted #id and will hence **generate the same selector o…

spring ...

The most obvious way to use spring is to call it explicitly:

spring rails console
spring rake db:migrate

Binstubs

Binstubs are wrapper scripts around executables. In Rails they live inside bin/. If you run spring binstub --all, your binstubs will be using Spring.

bin/rails console
bin/rake db:migrate

bundle exec rails ...

Bundle exec is inconsistent when it comes to spring. Some commands will use it, some won't.

```
bundle exec rails console # starts Spring…

When your model is using a callback like before_save or before_validation to calculate an aggregated value from its children, it needs to skip those children that are #marked_for_destruction?. Otherwise you will include children that have been ticked for deletion in a nested form.

Wrong way

class Invoice
  has_many :invoice_items
  accepts_nested_attributes_for :invoice_items, :allow_destroy => true # the critical code 1/2
  before_save :calculate_and_store_amount                              # the critical code 2/...

Background information about session storage in Rails

Rails has a default mechanism to store the session in the CookieStore. This is a cookie which holds the entire user session hash in the browser. This cookie is serialized, encoded with base64, and signed.

How Devise handles authentication

Devise uses this CookieStore. To track a users session, a salt is stored in the session cookie when a user logs in.
When a user logs out this CookieStore is overwrit…

An association defined with has_many :through will return the same record multiple times if multiple join models for the same record exist. 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).

Example

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
end

class Item < ActiveRecord::Base
  b...

In Ruby, classes and modules are called constants. This card explains how Ruby resolves the meaning of a constant.

The good

E. g. in the following example, Array could mean either Foo::Array or simply Array:

class Foo
  def list
    Array.new
  end
end

What Ruby does here is to see if the name Array makes sense inside of Foo::, and if that fails, resolves it to ::Array (without a namespace).

The bad

You might be surprised that these are all valid ways to reference Ruby's String class:

…