Step through migrations in Rails

tl;dr db:forward is the opposite of db:rollback

bin/rails db:migrate

runs all pending migrations.

If you want to migrate to a specific DB version, you can use

bin/rails db:migrate VERSION=20190730123456

which is rather cumbersome, especially if you only want to run one migration. Therefore you can use

bin/rails db:forward

You can combine it with variable STEP to execute multiple migrations.

bin/rails db:forward STEP=5
2019-08-02 by Mario Schüttel


rubocop 0.72.0 removes all Rails cops, since they “[…] want RuboCop to be focused just on Ruby […]” (and I agree with them), see PR #5976.

In order to use the Rails cops after rubocop 0.72.0, we need to add rubocop-rails gem and require it as a dependency in .rubocop.yml:

  - rubocop-rails
2019-07-29 by Mario Schüttel


Ruby enumerables respond to grep. You can use it to quickly filter a collection with a regular expression.

['Marta', 'Mario', 'Bill'].grep(/Ma/)
# => ['Marta', 'Mario']
2019-07-12 by Dimiter Petrov

Update nested maps in Elixir

Being a functional programming language, Elixir is a bit different from other languages when it comes to update a nested data structure.

For example, in javascript we could do:

data = {
  inner: {
    one: {
      a: 1
    two: {
      b: 45
} = 2

In Elixir you have to build a new map with the updated information, for example:

data = %{
  inner: %{
    one: %{
      a: 1
    two: %{
      b: 45

new_one = %{ | a: 2}
new_inner = %{data.inner | one: new_one}
new_data = %{data | inner: new_inner}

which is not very handy.

In other functional languages like Haskell, there are libraries like Lenses that aims to solve the problem. In Elixir the kernel have an put_in function that acts in a similar way:

data = put_in(data, [:inner, :one, :a], 2)

You can find other similar functions in the Kernel documentation

2019-06-13 by Tiziano Puppi

The query function in Ecto Repo (Elixir)

There is a hidden function in Ecto which is the function query in the Repo module.

This is a wrapper around the Ecto.Adapters.SQL.query and it is injected in the Repo module of your application. It can be called in this way:

MyApp.Repo.query("SELECT * FROM mytable")

This function can also take an array of query parameters like:

SELECT * FROM mytable
WHERE id = $1
""", [42])

This is quite handy when you need to debug in production, and there is no database client available.

2019-06-07 by Tiziano Puppi

Union, Difference and Intersect with Ruby Arrays


> [:a, :b, :c].union([:b, :c, :d])
 => [:a, :b, :c, :d]


> [:a, :b, :c].difference([:b, :c, :d])
 => [:a]


> [:a, :b, :c] & [:b, :c, :d]
 => [:b, :c]
2019-06-07 by Mario Schüttel

The sample output HTML element

There is an HTML element called <samp> meant for representing computer program output. The specification document has this humorous example:

<p>The computer said <samp>Too much cheese in tray
two</samp> but I didn't know what that meant.</p>