Today I Learned

A Hashrocket project

83 posts by viniciusnegrisolo @vfnegrisolo

Redirect outputs with subshell

Today I learned how to use a Subshell to redirect multiple outputs as a single one. This way I can run a bunch of commands and unify all the results as it was a single command.

(git diff --name-only; git diff --staged --name-only) | sort -u

The above example will show me a sorted list of changed files (including the staged ones) in a git project without duplicates.

Here’s another example:

$ echo "world"; echo "hello" | sort -u
#=> world
#=> hello

$ (echo "world"; echo "hello") | sort -u
#=> hello
#=> world


Today I learned that PostgreSQL INSERT/UPDATE/DELETE has a RETURNING clause that returns the computed values.

We can use it to get values that are generated by database such as sequences or defaults.

Check this out:



INSERT INTO users (email) VALUES ('');
-- INSERT 0 1

INSERT INTO users (email) VALUES ('') RETURNING *;
--  id |       email
-- ----+-------------------
--   2 |
-- (1 row)

h/t @joshuadavey

Vim undo tree

Vim has an undo tree and there’s two different ways to iterate with your changes.

One way to undo/redo changes is with: u and <Ctrl>r. This moves the current state based on the current branch in the tree.

The other way is using g- and g+. In this case it moves based on the timestamp of that change.

So here’s an example to follow:

  1. add a line: added_line_1
  2. update this line: updated_line_1
  3. undo this change: u
  4. add another line: added_line_2

If you run through these steps your undo tree will be something like:

 o  [3] "added_line_2" <= "current_state"
 | *  [2] "updated_line_1"
 *  [1] "added_line_1"
 *  [0] "empty_file"

Now, if you undo with u will be in this state:

 *  [3] "added_line_2"
 | *  [2] "updated_line_1"
 o  [1] "added_line_1" <= "current_state"
 *  [0] "empty_file"

Otherwise if you undo with g- will be in this part of the tree:

 *  [3] "added_line_2"
 | o  [2] "updated_line_1" <= "current_state"
 *  [1] "added_line_1"
 *  [0] "empty_file"

You can check gundo vim plugin for tree visualization.

Aggregate Expressions with FILTER in Postgresql

Today I learned to use FILTER on aggregate expressions such as COUNT in postgresql.

See this example:

SELECT u.login,
       COUNT(1) FILTER (WHERE r.language ilike 'ruby') as ruby,
       COUNT(1) FILTER (WHERE r.language ilike 'javascript') as js,
       COUNT(1) FILTER (WHERE r.language ilike 'elixir') as elixir,
       COUNT(1) as all_langs
FROM users u
LEFT JOIN repositories r ON ( = r.user_id)

-- login | ruby | js | elixir | all_langs
-- bill  |    5 |  2 |      3 |         15
-- karen |    2 |  7 |      4 |         19
-- bob   |    9 |  1 |      2 |         23

h/t @joshuadavey

Using Ruby Hash in `gsub`

Ruby String#gsub method also accepts a hash as second argument. This is perfect for some situations like mapping a phone-word to phone number:

phonewords = {
  'a' => 2, 'b' => 2, 'c' => 2,
  'd' => 3, 'e' => 3, 'f' => 3,
  'g' => 4, 'h' => 4, 'i' => 4,
  'j' => 5, 'k' => 5, 'l' => 5,
  'm' => 6, 'n' => 6, 'o' => 6,
  'p' => 7, 'q' => 7, 'r' => 7, 's' => 7,
  't' => 8, 'u' => 8, 'v' => 8,
  'w' => 9, 'x' => 9, 'y' => 9, 'z' => 9,
phone = "1-800-map-gsub"
puts phone.gsub(/[a-z]/, phonewords)
# => 1-800-627-4782

h/t @joshuadavey

Rails Enums and PostgreSQL Enums

Today I learned that Rails Enum works pretty well with PostgreSQL Enum.

Enum in PostgreSQL work as a new type with restricted values and if you need to guarantee data integrity there’s no best way to do that than in your database. Here is how we create a new enum in PostgreSQL and use in a new column:

class AddEnumToTrafficLights < ActiveRecord::Migration
  def up
    ActiveRecord::Base.connection.execute <<~SQL
      CREATE TYPE color AS ENUM ('red', 'green', 'blue');

    add_column :traffic_lights, :color, :color, index: true

  def down
    remove_column :traffic_lights, :color

    ActiveRecord::Base.connection.execute <<~SQL
      DROP TYPE color;

In Rails models I prefer to use Hash syntax when mapping Ruby symbols to database values.

class TrafficLight < ApplicationRecord
  enum color: {
    red: 'red',
    green: 'green',
    blue: 'blue',

And now Enum in action:

#=> {
#=>   "red"=>"red",
#=>   "green"=>"green",
#=>   "blue"=>"blue"
#=> }!
#=> "blue"

h/t @ joshuadavey

Rails Enum with prefix/suffix


Today I learned that Rails 5 released new options for [enum][] definition: _prefix and _suffix.

class Conversation < ActiveRecord::Base
  enum status:          [:active, :archived], _suffix: true
  enum comments_status: [:active, :inactive], _prefix: :comments

conversation.archived_status? # => false

conversation.comments_active? # => false

h/t @joshuadavey

Dealing with exit signals on Ruby

When working on CLIs it’s useful to treat if the application exists. Image you have a long application and even if the user hits a <Ctrl+c> to kill the process you still want to display the results so far.

There is a Ruby Kernel method at_exit that runs the block passed to it if the current process exists. Here is an example:

# print_at_exit.rb
at_exit { puts "Come back later!" }
puts "sleeping for 5 secs"
sleep 5

If you run this file and wait it until it finishes you’ll get:

ruby print_at_exit.rb                                                                                                                {130}
#=> sleeping for 5 secs
#=> Come back later!

But now, if you run it and hit <Ctrl+c> for finishing the process earlier you’ll get:

ruby print_at_exit.rb
#=> sleeping for 5 secs
#=> Come back later!
#=> print_at_exit.rb:6:in `sleep': Interrupt
#=>         from print_at_exit.rb:6:in `<main>'

h/t @nikkypx

Ruby defined? method evaluation

Today I learned that Ruby Object method defined? evaluates if the argument is anything recognizable, not just if the variable are set. Here are some examples:

undefined_foo #=> NameError: undefined local variable or method `undefined_foo'
defined? undefined_foo #=> nil

defined? nil #=> "nil"

defined? "foo" #=> "expression"
defined? [] #=> "expression"
defined? def x; end #=> "expression"
defined? class Foo; end #=> "expression"

defined? @foo = :bar #=> "assignment"

@foo = :bar
defined? @foo #=> "instance-variable"

foo = :bar
defined? foo #=> "local-variable"

def foo; end
defined? foo() #=> "method"

class Foo; end
defined? Foo #=> "constant"

h/t @VidalEkechukwu

One query with Select In

Today I learned that if you pass an ActiveRecord::Relation as param to where Rails will create a single query using IN and an inner SELECT:

=>  Repository Load (1.1ms)
        SELECT "repositories".*
        FROM "repositories"
        WHERE "repositories"."user_id" IN (
          SELECT "users"."id"
          FROM "users"
          WHERE "users"."status" == 1

h/t @mattpolito

Ember nested routes and it's active class

Ember adds automagically a css class active to all link-to elements under the current route.

So let’s say you have this in your router:

// app/router.js {
  this.route('blog', function() {
    this.route('post', { path: ':slug' });

If you access a page like http://localhost:4200/blog/last-post, all the following links will have the active css class.

{{#link-to 'blog'}}Blog{{/link-to}}
{{#link-to 'blog' post}}{{}}{{/link-to}}
<a id="ember661" href="/blog" class="ember-view active">Blog</a>
<a id="ember663" href="/blog/last-post" class="ember-view active">Last Post</a>

So far so good, but now, if by any reason you just want either child link or parent link with the active class you can change the parent link-to to use .index.

{{#link-to 'blog.index'}}Blog{{/link-to}}

So both blog and blog.index links to the same url, but they act differently with active class.

ActiveJobs callbacks with conditionals

Today I learned that ActiveJobs callbacks accept filters, so it’s easy to do conditionals like:

class SyncUserJob < ApplicationJob
  queue_as :default

  after_perform(if: :recursive) do
    SyncOrganizationJob.perform_later(user, recursive: true)

  attr_accessor :user, :recursive

  def perform(access_token, recursive: false)
    @access_token = access_token
    @recursive    = recursive
    @user         = sync_user

  def sync_user

More context with git diff

So if you want to see more (or less) context when running a git diff command you can use -U<number_of_lines>.

Here it is a regular git diff (3 lines as default):

vnegrisolo@home:~/my-app(master)% git diff
index modified: app/jobs/github/sync_organization_job.rb
@ sync_organization_job.rb:24 @ module Github
      organization.login       = data['login']
      organization.description = data['description']
      organization.avatar_url  = data['avatar_url']
-      organization.user        = @user
+      organization.user        = user
      recursive_sync(organization) if @recursive

Now just showing the changed lines:

vnegrisolo@home:~/my-app(master)% git diff -U0
index modified: app/jobs/github/sync_organization_job.rb
@ sync_organization_job.rb:27 @ module Github
-      organization.user        = @user
+      organization.user        = user

Finally with 5 lines of context instead of 3:

vnegrisolo@home:~/my-app(master)% git diff -U5
index modified: app/jobs/github/sync_organization_job.rb
@ sync_organization_job.rb:22 @ module Github
      organization = Organization.find_or_initialize_by(github_id: data['id'])

      organization.login       = data['login']
      organization.description = data['description']
      organization.avatar_url  = data['avatar_url']
-      organization.user        = @user
+      organization.user        = user
      recursive_sync(organization) if @recursive

Ruby Method arity

Today I learned that Method#arity does not consider blocks.

Also it returns negative with splat arguments.

Finally, all keyword arguments counts 1.

class Foo
  def one; end
  def two(a, b); end
  def three(a, *b); end
  def four(a, &b); end
  def five(a, b:, c:); end

puts Foo.instance_method(:one).arity
# => 0
puts Foo.instance_method(:two).arity
# => 2
puts Foo.instance_method(:three).arity
# => -2
puts Foo.instance_method(:four).arity
# => 1
puts Foo.instance_method(:five).arity
# => 2

h/t @higgaion

Rails reset counter caches

Today I learned that Rails has an easy way to reset counter caches

You just need to call reset_counters(id, *counters) method

irb> Organization.reset_counters(4, :repositories)
  Organization Load (0.3ms)  SELECT  "organizations".* FROM "organizations" WHERE "organizations"."id" = $1 LIMIT $2  [["id", 4], ["LIMIT", 1]]
   (0.5ms)  SELECT COUNT(*) FROM "repositories" WHERE "repositories"."organization_id" = $1  [["organization_id", 4]]
  SQL (2.1ms)  UPDATE "organizations" SET "repositories_count" = 6 WHERE "organizations"."id" = $1  [["id", 4]]
=> true

Rails autoload paths

Today I learned that I can see Rails autoload_paths running:

puts ActiveSupport::Dependencies.autoload_paths

You can always add more paths with:

# config/application.rb
config.autoload_paths << Rails.root.join('lib')

By the way, that’s why concerns (model/controller) do not have Concern as namespace.

Ember queryParams default value has to be static

Today I learned that Ember queryParams do not work with computed properties. They intend to be static values and here is an issue discussion about that.

So this code will not work properly:

export default Ember.Controller.extend({
  queryParams: ['start'],
  start: new Date().getFullYear() + '-' + (new Date().getMonth() + 1) + '-' + new Date().getDate();
export default Ember.Route.extend({
  queryParams: {
    start: { refreshModel: true }
  model(params) {
    console.log('start', params.start);

In this example the default value is cached and if you try to change it manually, or use a computed property with volatile, you’ll see a delegate is not a function error message.

Date javascript

Date javascript function on chrome is weird:

new Date('2016-10-3')
Mon Oct 03 2016 00:00:00 GMT-0400 (EDT)
new Date('2016-10-10')
Sun Oct 09 2016 20:00:00 GMT-0400 (EDT)

Why? Anyway, this is my “alternative” approach for that:

new Date('2016-10-10 0:0')
Mon Oct 10 2016 00:00:00 GMT-0400 (EDT)
new Date('2016-10-3 0:0')
Mon Oct 03 2016 00:00:00 GMT-0400 (EDT)

Custom QUnit assertions

QUnit is the test framework on Ember land. It’s really good, but there are just 14 assertions to be used. We have ok() and notOk() so it seems that we can do whatever we want, but when some test fails the message are not good enough.

A simple string includes example:

// tests/acceptance/home-test.js
assert.ok(homePage.title.includes('Welcome to TIL'));
// => failed, expected argument to be truthy, was: false

To improve that I created this new custom assertion:

// tests/helpers/custom-asserts.js
import QUnit from 'qunit';

QUnit.assert.includes = function(text, expected) {
  let message = 'expected: "' + expected + '" to be included on: "' + text + '"';
  this.ok(text.includes(expected), message);

And imported in:

// tests/helpers/module-for-acceptance.js
import '../helpers/custom-asserts';

And here it is the new message on error:

// tests/acceptance/home-test.js
assert.includes(homePage.title, 'Welcome to TIL')
// => expected: "Welcome to TIL" to be included on: "Welcome to Today I Learned!"

Rails and HttpAuthentication Token

Rails has some controller helper modules for authentication:

So you can have on your controller like that:

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  include UserAuthentication

  before_action :authenticate, only: %i(show)

  def show
    render json: current_user, status: :ok

And a controller concern like this:

# app/controllers/concerns/user_authentication.rb
module UserAuthentication
  # you might need to include:
  # include ActionController::HttpAuthentication::Token::ControllerMethods

  def authenticate
    head :forbidden unless current_user

  def current_user
    @current_user ||= authenticate_or_request_with_http_token do |token|
      Session.find_by(token: token).try(:user)

Then your controller will read and parse the token from the header:

  headers: {
    "HTTP_AUTHORIZATION"=>'Token token="82553421c8f4e5e34436"'

Rails 5 token ActiveModel type

Rails 5 has a new ActiveModel type token. To define it you can use: has_scure_token.

So I had this class:

class Session < ApplicationRecord
  belongs_to :user

  before_validation(on: :create) do
    self.auth_token = SecureRandom.urlsafe_base64(24)

And then I started to use the new has_secure_token and the code is way simpler now:

class Session < ApplicationRecord
  belongs_to :user
  has_secure_token :auth_token

If you use generators for models you can run:

rails generate model session auth_token:token

By the way the current implementation uses SecureRandom.base58(24), so it is url safe.

Print call stack on Javascript

If you want to print the javascript call stack you can use:

console.log(new Error().stack);

Here it is an example:

function firstFunction() {
function secondFunction() {
function thridFunction() {
    console.log(new Error().stack);

//=> Error
//    at thridFunction (<anonymous>:2:17)
//    at secondFunction (<anonymous>:5:5)
//    at firstFunction (<anonymous>:8:5)
//    at <anonymous>:10:1

h/t @rondale_sc

Elixir with macro `<-` and `=`

So Elixir with macro accepts Matching clauses <- and Bare expressions =. Both match patterns and do not leak variables assigned inside these structures.

with {:ok, width} <- Map.fetch(%{width: 10}, :width),
  do: {:ok, 2 * width}
#=> {:ok, 20}

with {:ok, width} = Map.fetch(%{width: 10}, :width),
  do: {:ok, 2 * width}
#=> {:ok, 20}

So what is the difference between these?

When a match fails, matching clauses <- returns failed result but bare expressions = raises a MatchError:

with {:ok, width} <- Map.fetch(%{height: 10}, :width),
  do: {:ok, 2 * width}
#=> :error

with {:ok, width} = Map.fetch(%{height: 10}, :width),
  do: {:ok, 2 * width}
#=> ** (MatchError) no match of right hand side value: :error

Another difference is that when guard is only available for matching clause.

with {:ok, width} when is_number(width) <- Map.fetch(%{width: 10}, :width),
  do: {:ok, 2 * width}
#=> {:ok, 20}

Rails 5 - Array Inquirer

Rails 5 has a new wrapper class for Array called ArrayInquirer the same way StringInquirer does, but with arrays.

irb> colors = [:blue, 'green'].inquiry
=> [:blue, "green"]
irb> colors.class
=> ActiveSupport::ArrayInquirer

This will create nice question like methods such as:

irb> colors = [:blue, 'green'].inquiry
=> [:blue, "green"]
=> true
=> true
=> false

It also creates the method any? for you:

irb> colors = [:blue, 'green'].inquiry
=> [:blue, "green"]
irb> colors.any?('blue', :yellow)
=> true
irb> colors.any?('red', :yellow)
=> false

By the way we use StringInquirer already:

irb> Rails.env
=> "development"
irb> Rails.env.class
=> ActiveSupport::StringInquirer
irb> Rails.env.development?
=> true


Ember prevent nested actions to be called

If you have an element that is affected by multiple actions you can use bubbles=false to prevent that action to bubble up to the next on the hierarchy.

<div class="container-fluid" {{action 'hideNav'}}>
  <div class="navbar-header">
    <button {{action 'toggleNav' bubbles=false}} type="button" class="navbar-toggle collapsed" aria-expanded="false">
      <span class="sr-only">Toggle navigation</span>
      <span class="icon-bar"></span>
      <span class="icon-bar"></span>
      <span class="icon-bar"></span>
    {{link-to name 'index' class="navbar-brand"}}

  <div class="collapse navbar-collapse">

Rails 5 db schema and its indexes

Rails 5 changed the way they register db indexes on db/schema.rb file. Now it’s nested to create_table method.

rails 5

create_table "users", force: :cascade do |t|
  t.index ["username"],
    name: "index_users_on_username", using: :btree

rails 4

add_index "users", ["username"],
  name: "index_users_on_username", using: :btree

Foreign keys are registered the same way as before:

ActiveRecord::Schema.define(version: 20160524224020) do
  add_foreign_key "sessions", "users"

Rails 5 - Use limit on ActiveRecords with OR

Rails 5 comes released a new ActiveRecord or but it still have some constraints you should know.

limit, offset, distinct should be on both queries, otherwise:

irb(main):001:0> Session.where(user_id: 1).or(
                   Session.where(provider: 'github').limit(10)
=> ArgumentError: Relation passed to #or must be structurally compatible.
   Incompatible values: [:limit]

You need to add limit to both sides of the query, like:

irb(main):002:0> puts Session.where(user_id: 1).limit(10).or(
                   Session.where(provider: 'github').limit(10)
=> "SELECT  "sessions".*
    FROM "sessions"
    WHERE ("sessions"."user_id" = 1 OR "sessions"."provider" = 'github')
    LIMIT 10"

Does CoffeeScript has a ternary operator?

Javascript has a ternary operator condition ? expr1 : expr2 like:

var x = true;
x ? 'yes' : 'no'
//=> "yes"
x = false;
x ? 'yes' : 'no'
//=> "no"

What about CoffeeScript?
x ? 'yes' : 'no'

will be compiled to the following:

if (typeof x !== 'undefined' && x !== null) {
} else {
  ({ 'yes': 'no' });

Why? Because of The Existential Operator

So there is no ternary operator on CoffeeScript. The best you can do is to have an one-line if-else statement:

if x then 'yes' else 'no'

h/t @mwoods79

Vim tree view mode

In folder/file structure if you type i you’ll change the way you see the folders and files.

There are 4 different view modes and one of them is something like that:

| app/
| bin/
| config/
| | environments/
| | | development.rb
| | | production.rb
| | | test.rb
| | initializers/
| | locales/
| | application.rb
| | boot.rb
| | database.yml
| | environment.rb
| | routes.rb
| db/

So yes, We don’t need nerdtree

Rails 5 API and CORS

So if you have your frontend and backend on different domains you’ll need to allow CORS (cross-origin HTTP request).

Rails 5 with --api mode will prepare the setup for you. You just need to uncomment the following:

# Gemfile
gem 'rack-cors'
# config/initializers/cors.rb
Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins 'localhost:4200'
    resource '*',
      headers: :any,
      methods: %i(get post put patch delete options head)