Mechanics of Migrations

Objective

1. Create, connect to, and manipulate a SQLite database using ActiveRecord.

Setup

We're going to use the activerecord gem to create a mapping between our database and model. Clone down this lesson and code-along to get to the solution.

Migrations

From the RailsGuides section on Migrations:

Migrations are a convenient way for you to alter your database in a structured and organized manner. You could edit fragments of SQL by hand but you would then be responsible for telling other developers that they need to go and run them. You’d also have to keep track of which changes need to be run against the production machines next time you deploy.

Migrations also allow you to describe these transformations using Ruby. The great thing about this is that (like most of Active Record’s functionality) it is database-independent: you don’t need to worry about the precise syntax of CREATE TABLE any more than you worry about variations on SELECT * (you can drop down to raw SQL for database-specific features). For example, you could use SQLite3 in development, but MySQL in production.

Another way to think of migrations is like version control for your database. You might create a table, add some data to it, and then make some changes to it later on. By adding a new migration for each change you make to the database, you won't lose any data you don't want to, and you can easily revert changes.

Executed migrations are tracked by ActiveRecord in your database so that they aren't used twice. Using the migrations system to apply the schema changes is easier than keeping track of the changes manually and executing them manually at the appropriate time.

Setting Up Your Migration

1. Create a directory called db at the top level of the lesson's directory. Then, within the db directory, create a migrate directory. The mkdir command is the appropriate tool to use here.

2. In the db/migrate directory, create a file called 01_create_artists.rb (we'll talk about why we added the 01 later).

mechanics-of-migrations-v-000/
  config/
    environment.rb
  db/
    migrate/
      01_create_artists.rb
  spec/
    artist_spec.rb
    spec_helper.rb
  .gitignore
  .learn
  .rspec
  artist.rb
  CONTRIBUTING.md
  Gemfile
  Gemfile.lock
  LICENSE.md
  Rakefile
  README.md

With the file created, we'll need to add in the migration code:

# db/migrate/01_create_artists.rb

class CreateArtists < ActiveRecord::Migration[5.2]
  def up
  end

  def down
  end
end

Active Record 5.x Migration Syntax Update

IMPORTANT: Active Record is primarily used in Rails applications and as of Active Record 5.x, we must specify which version of Rails the migration was written for, even in situations like this lab where we do not have Rails configured.

This lesson was originally created with gem versions that support Rails 5.2, so we need to make have our CreateArtist migration inherit from ActiveRecord::Migration[5.2].

Don't worry too much about this until you get to the Rails section. Until then, if you encounter an error like this...

Caused by:
StandardError: Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for:

  class CreateArtists < ActiveRecord::Migration[5.2]

...simply add [5.2] or whatever number is displayed to the end of ActiveRecord::Migration in your migration file, exactly as the error message instructs.

Active Record Migration Methods: up, down, change

Here we're creating a class called CreateArtists that inherits from ActiveRecord's ActiveRecord::Migration module. Within the class, we have an up method to define the code to execute when the migration is run and a down method to define the code to execute when the migration is rolled back. Think of it like "do" and "undo."

Another method is available to use besides up and down: change, which is more common for basic migrations.

# db/migrate/01_create_artists.rb

class CreateArtists < ActiveRecord::Migration[5.2]
  def change
  end
end

From the Active Record Migrations RailsGuide:

The change method is the primary way of writing migrations. It works for the majority of cases, where Active Record knows how to reverse the migration automatically

Let's take a look at how to finish off our CreateArtists migration, which will generate our artists table with the appropriate columns.

Creating a Table

Remember how we created a table using SQL with ActiveRecord?

NOTE: Recall, we can do this with IRB: irb -r active_record

First, we connect to a database, then write the necessary SQL to create the table. So, first, we'd have to connect to a database:

ActiveRecord::Base.establish_connection(
  :adapter => "sqlite3",
  :database => "db/artists.sqlite"
)

Then write some SQL to create the table:

sql = <<-SQL
  CREATE TABLE IF NOT EXISTS artists (
  id INTEGER PRIMARY KEY,
  name TEXT,
  genre TEXT,
  age INTEGER,
  hometown TEXT
  )
SQL

ActiveRecord::Base.connection.execute(sql)

Using migrations, we will still need establish Active Record's connection to the database, but we no longer need the SQL! Instead of dealing with SQL directly, we provide the migrations body (in Ruby) and Active Record takes care of creating complex SQL commands. This is less error-prone and much easier to read.

As a bonus, migrations, when paired with version control (git), create a record of changes to the database. If we just ran a command on the database, there's no undo, no opportunity for peer review and pull request behaviors. Migrations help programmers think about purpose not syntax.

Since we still need to connect to the database, let's make the connection inside config/environment.rb:

# config/environment.rb
require 'rake'
require 'active_record'
require 'yaml/store'
require 'ostruct'
require 'date'

require 'bundler/setup'
Bundler.require

# put the code to connect to the database here
ActiveRecord::Base.establish_connection(
  :adapter => "sqlite3",
  :database => "db/artists.sqlite"
)

require_relative "../artist.rb"

Reminder: The environment.rb file commonly contains things we want to read and make available to our Ruby application when it starts. It isn't necessary that you totally grasp all the parts of this file, but looking through it with this in mind, you might be able to gather what is happening: some gems, including active_record are required; something happens with Bundler; our database connection is established; the artist.rb file is read.

With the connection to the database configured, we should have access to ActiveRecord::Migration, and can create tables using only Ruby!

# db/migrate/01_create_artists.rb
def change
  create_table :artists do |t|
  end
end

Here we've added the create_table method and passed the name of the table we want to create as a symbol. Pretty simple, right? Other methods we can use here are things like remove_table, rename_table, remove_column, add_column and others. See this list for more.

No point in having a table that has no columns in it, so let us add a few:

# db/migrate/01_create_artists.rb

class CreateArtists < ActiveRecord::Migration[5.2]
  def change
    create_table :artists do |t|
      t.string :name
      t.string :genre
      t.integer :age
      t.string :hometown
    end
  end
end

Looks a little familiar? On the left we've given the data type we'd like to cast the column as, and on the right, we've given the name we'd like to give the column. The only thing that we're missing is the primary key. Active Record will generate that column for us, and for each row added, a key will be auto-incremented.

And that's it! You've created your first Active Record migration. Next, we're going to see it in action!

Running Migrations

The simplest way is to run our migrations through a Rake task that we're given through the activerecord gem. How do we access these?

Run rake -T to see the list of commands we have.

Note: If you get an error when trying to run rake commands, you may have a newer version of rake already installed compared to this lesson, causing a conflict. To avoid this error, run bundle exec rake -T. Adding bundle exec indicates that you want rake to run within the context of this lesson's bundle (defined in the Gemfile), not the default version of rake you have installed globally on your computer.

Let's look at the Rakefile. The commands listed when running rake -T are made available as Rake tasks through require 'sinatra/activerecord/rake'.

Now take a look again at environment.rb, which our Rakefile also requires:

# config/environment.rb

require 'bundler/setup'
Bundler.require

ActiveRecord::Base.establish_connection(
  :adapter => "sqlite3",
  :database => "db/artists.sqlite"
)

This file is requiring the gems in our Gemfile and giving our program access to them. We're using the establish_connection method from ActiveRecord::Base to connect to our artists database, which will be created in the migration via SQLite3 (the adapter).

After we've added the above code to config/environment.rb, it's time to run rake db:migrate.

Note: Here again, if you encounter an error after running rake db:migrate, try running bundle exec rake db:migrate.

Take a look at artist.rb. Let's create an Artist class.

# artist.rb

class Artist
end

Next, we'll extend the class with ActiveRecord::Base

# artist.rb

class Artist < ActiveRecord::Base
end

To test our newly-created class out, let's use the rake task rake console (or bundle exec rake console), which we've created in the Rakefile.

Try Out The Following:

Check that the class exists:

Artist
# => Artist (call 'Artist.connection' to establish a connection)

View the columns in its corresponding table in the database:

Artist.column_names
# => ["id", "name", "genre", "age", "hometown"]

Instantiate a new Artist named Jon, set his age to 30, and save him to the database:

a = Artist.new(name: 'Jon')
# => #<Artist id: nil, name: "Jon", genre: nil, age: nil, hometown: nil>

a.age = 30
# => 30

a.save
# => true

The .new method creates a new instance in memory, but for that instance to persist, we need to save it. If we want to create a new instance and save it all in one go, we can use .create.

Artist.create(name: 'Kelly')
# => #<Artist id: 2, name: "Kelly", genre: nil, age: nil, hometown: nil>

Return an array of all Artists from the database:

Artist.all
# => [#<Artist id: 1, name: "Jon", genre: nil, age: 30, hometown: nil>,
 #<Artist id: 2, name: "Kelly", genre: nil, age: nil, hometown: nil>]

Find an Artist by name:

Artist.find_by(name: 'Jon')
# => #<Artist id: 1, name: "Jon", genre: nil, age: 30, hometown: nil>

There are several methods you can now use to create, retrieve, update, and delete data from your database, and a whole lot more.

Take a look at these CRUD methods, and play around with them.

Using Migrations To Manipulate Existing Tables

Here is another place where migrations really shine. Let's add a favorite_food column to our artists table. Remember that Active Record keeps track of the migrations we've already run, so adding the new code to our 01_create_artists.rb file won't work. Since we aren't rolling back our previous migration (or dropping the entire table), the 01_create_artists.rb migration won't be re-executed when we run rake db:migrate again. Generally, the best practice for database management (especially in a production environment) is creating new migrations to modify existing tables. That way, we'll have a clear, linear record of all of the changes that have led to our current database structure.

To make this change we're going to need a new migration, which we'll call 02_add_favorite_food_to_artists.rb.

# db/migrate/02_add_favorite_food_to_artists.rb

class AddFavoriteFoodToArtists < ActiveRecord::Migration[5.2]
  def change
    add_column :artists, :favorite_food, :string
  end
end

Pretty awesome, right? We just told Active Record to add a column to the artists table called favorite_food and that it will contain a string.

Notice how we incremented the number in the file name there? Imagine for a minute that you deleted your original database and wanted to execute the migrations again. Active Record is going to execute each file, but it does so in alpha-numerical order. If we didn't have the numbers, our add_column migration would have tried to run first ([a]dd_favorite... comes before [c]reate_artists...), and our artists table wouldn't have even been created yet! So we used some numbers to make sure the migrations execute in order. In reality, our two-digit system is very rudimentary. As you'll see later on, frameworks like Rails have generators that create migrations with very accurate timestamps, so you'll never have to worry about hand-numbering.

Now that you've saved the migration, go back to the terminal to run rake db:migrate.

Awesome! Now go back to the console with the rake console command, and check it out:

Artist.column_names
# => ["id", "name", "genre", "age", "hometown", "favorite_food"]

Great!

Nope- wait. Word just came down from the boss- you weren't supposed to ship that change yet! OH NO! No worries, we'll roll back to the first migration.

Run rake -T. Which command should we use?

Then drop back into the console to double check:

Artist.column_names
# => ["id", "name", "genre", "age", "hometown"]

Oh good, your job is saved. Thanks, Active Record! When the boss says it's actually time to add that column, you can just run rake db:migrate again!

Woohoo!

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