muneerabaz/w10d05-rails-relationships

Rails Relationships

Learning Objectives

• Understand how to make migrations that reference other models
• Set up relationships in models using belongs_to and has_many
• Use ActiveRecord models to query and create related entities
• Learn how shallow routes support related models

Referenced Migrations

Lets say we have an app where we are storing information about music artists and their work. We can start by generating a model for an artist with:

rails g model Artist name:string genre:string

This will create an Artist model and a migration to add the table to our database. If we want to then add a table where we keep track of each artist's albums, we'll want to generate a new model for Albums. In the SQL world we learned about foreign keys and how they keep track of a primary key that the entry relates to. For example an Album can have an artist_id that references back to an artist. In rails we can add this reference when we generate the model:

rails g model Album name:string release_year:integer artist:references

This will create a migration that looks like:

class CreateAlbums < ActiveRecord::Migration[5.2]
  def change
    create_table :albums do |t|
      t.string :name
      t.integer :release_year
      #this line will add our foreign key for artist_id
      t.references :artist, foreign_key: true 

      t.timestamps
    end
  end
end

belongs_to

Generating that model will also create a model file that looks like:

class Album < ApplicationRecord
  belongs_to :artist
end

A line is added that tells our model that there is a relationship to another table. This does a few things.

1. It makes it so when we create an album, we need to specify an artist for it.

kesha = Artist.create({
  name: "Ke$ha",
  genre: "pop"
})

rainbow = Album.create({
  name: "Rainbow",
  release_year: 2017,
  artist: kesha # we need to specify an artist here or there will be errors!
})

2. It allows us to access our artist from our album

rainbow.artist # will give us kesha

has_many

At this point, we may also want to be able to access all of the albums from an artist. To add these methods to our Artist, we need to edit our artist model:

class Artist < ApplicationRecord
  has_many :albums
end

Now we can access albums from any artist.

Artist.first.albums # will give us back all of the albums associated with the first artist

We can also create albums through our artist

kesha = Artist.find_by({ name: "Ke$ha" })
kesha.albums.create({
  name: "Animal",
  release_year: 2010,
  artist: kesha
}) # will create the "Animal" album with the artist "Ke$ha"

Rails relational routing

Setting up routes can be challenging. There are a lot of ways to do so and there often isn't always a "right way". Its always good to think about your application and what you need from it and go through your options.

For example, for our music app we could:

1. Have distinct seperate routes for artists and albums:

Rails.application.routes.draw do
  resources :artists
  resources :albums
end

which would give us:

method	route	result
GET	/artists	get all artists
POST	/artists	create an artist
GET	/artists/:id	get an artist by id
PUT	/artists/:id	update an artist
DELETE	/artists/:id	delete an artist
GET	/albums	get all albums
POST	/albums	create an album
GET	/albums/:id	get an album by id
PUT	/albums/:id	update an album
DELETE	/albums/:id	delete an album

2. Nest albums completely under artists:

Rails.application.routes.draw do
  resources :artists do
    resources :albums
  end
end

which would give us:

method	route	result
GET	/artists	get all artists
POST	/artists	create an artist
GET	/artists/:id	get an artist by id
PUT	/artists/:id	update an artist
DELETE	/artists/:id	delete an artist
GET	/artists/:artist_id/albums	get albums by an artist
POST	/artists/:artist_id/albums	create an album belonging to an artist
GET	/artists/:artist_id/albums/:id	get an album by artist id and album id
PUT	/artists/:artist_id/albums/:id	update an album by artist id and album id
DELETE	/artists/:artist_id/albums/:id	delete an album by artist id and album id

3. Use what we call "shallow" routes for the albums

Rails.application.routes.draw do
  resources :artists do
    resources :albums, shallow: true
  end
end

which would give us:

method	route	result
GET	/artists	get all artists
POST	/artists	create an artist
GET	/artists/:id	get an artist by id
PUT	/artists/:id	update an artist
DELETE	/artists/:id	delete an artist
GET	/artists/:artist_id/albums	get albums by an artist
POST	/artists/:artist_id/albums	create an album belonging to an artist
GET	/albums/:id	get an album by id
PUT	/albums/:id	update an album
DELETE	/albums/:id	delete an album

The last one here is good because it stops our routes from getting too long. I like to do a bit of a mixture where I use shallow routes, but also add in an index for albums not scoped to artists. This way I have a route to see all of the albums, not just by artist.

Rails.application.routes.draw do
  resources :artists do
    resources :albums, shallow: true
  end

  get "/albums", to: "albums#index"
end

which would give us:

method	route	result
GET	/artists	get all artists
POST	/artists	create an artist
GET	/artists/:id	get an artist by id
PUT	/artists/:id	update an artist
DELETE	/artists/:id	delete an artist
GET	/artists/:artist_id/albums	get albums by an artist
POST	/artists/:artist_id/albums	create an album belonging to an artist
GET	/albums/:id	get an album by id
PUT	/albums/:id	update an album
DELETE	/albums/:id	delete an album
GET	/albums	get all albums