1 to Many Associations

Objectives

• Implement one to many relationships through models in Rails
• Use the collection_select form helper to display a collection of associated items

Let's create a minimal rails app so that we can demonstrate the use of associations inside the rails app.

Rails gives us a few tools to make associations easy inside the controllers and views (in addition to active record)

Create the app:

rails new parklist -d postgresql

What we need

• Models
  • Park
  • Ranger
• Association
  • Park has_many Rangers
  • Ranger belongs_to a Park

• Views

  • parks#new

  • parks#show

    • list all rangers with a specific park

  • rangers#new - add/remove rangers select

  • rangers#show

• Controllers

  • parks#new

  • parks#show

    • list all rangers with a specific park

  • rangers#new - get a list of all parks here

  • rangers#show

• Routes

Generating models / databse migrations

Review of Parks

rails generate migration parks

create_table :parks do |t|
  t.string :name
  t.text :description
  t.timestamps
end

touch app/models/park.rb (model filenames are singular)

class Park < ActiveRecord::Base
end

rails generate migration rangers

create_table :rangers do |t|
  t.string :name
  t.references :park
  t.timestamps
end

Run the db scripts

rails db:create
rails db:migrate

app/models/ranger.rb

touch app/models/ranger.rb (model filenames are singular)

class Ranger < ActiveRecord::Base
  belongs_to :park
end

Change the model file to associate Park with Ranger:

app/models/park.rb

has_many :ranger

Check your work on the command line

rails console

yellowstone = Park.new(name: "yellowstone", description: "pretty cool")
yellowstone.save

ranger = Ranger.new(name: "roger", park: yellowstone)
ranger.save

Now we have a set of related records. This active record query should work:

Ranger.first.park.name

Check your work in psql

rails dbconsole

SELECT * FROM rangers;

Set up our requests:

Nested route

  root 'parks#index'
  get '/parks' => 'parks#index', as: 'parks'
  get '/parks/new' => 'parks#new', as: 'new_park'
  post '/parks' => 'parks#create'
  get '/parks/:id' => 'parks#show' , as: 'park'
  get '/parks/:id/edit' => 'parks#edit', as: 'edit_park'
  patch '/parks/:id' => 'parks#update'
  delete '/parks/:id' => 'parks#destroy'


  get '/rangers' => 'rangers#index', as: 'rangers'
  get '/rangers/new' => 'rangers#new', as: 'new_ranger'
  post '/rangers' => 'rangers#create'
  get '/rangers/:id' => 'rangers#show' , as: 'ranger'
  get '/rangers/:id/edit' => 'rangers#edit', as: 'edit_ranger'
  patch '/rangers/:id' => 'rangers#update'
  delete '/rangers/:id' => 'rangers#destroy'


  get '/parks/:park_id/rangers' => 'rangers#index', as: 'park_rangers'
  get '/parks/:park_id/rangers/new' => 'rangers#create', as: 'new_park_ranger'
  post '/parks/:park_id/rangers' => 'rangers#create'

Test the routes they produce: rake routes

Make the controllers:

app/controllers/parks_controller.rb

class ParksController < ApplicationController

  def new
  end

  def create
    @park = Park.new(park_params)

    @park.save
    redirect_to @park
  end

  def show
    @park = Park.find(params[:id])
  end

private

  def park_params
    params.require(:park).permit(:name, :description)
  end

end

app/controllers/rangers_controller.rb

class RangersController < ApplicationController

  def new
    @parks = Park.all
  end

  def create
    @ranger = Ranger.new(ranger_params)

    @ranger.save
    redirect_to @ranger
  end

  def show
    # deal with the case that we are trying to get a ranger for a park that doesn't exist

    @ranger = Ranger.find(params[:id])

    if params[:park_id].to_i != @ranger.park.id
      # do something
    end
  end

private

  def ranger_params
    params.require(:ranger).permit(:name, :park_id)
  end
end

views

The app/views/parks/new.html.erb file looks the same:

app/views/parks/new.html.erb

<%= form_with scope: :park, url: parks_path, local: true do |form| %>

  <p>
    <%= form.label :name %><br>
    <%= form.text_field :name %>
  </p>

  <p>
    <%= form.label :description %><br>
    <%= form.text_area :description %>
  </p>

  <p>
    <%= form.submit %>
  </p>
<% end %>

rangers#new

creating the ranger -> park association:

• Prepare in the ranger#newcontroller: make an instance variable with @parks

  @parks = Park.all
  

  Create a pre-populated select tag:

  <%= f.collection_select :park_id, @parks, :id, :name %>
  

All together it should look like this app/views/rangers/new.html.erb

app/views/rangers/new.html.erb

<%= form_with scope: :ranger, url: rangers_path, local: true do |form| %>

  <%= form.collection_select :park_id, @parks, :id, :name %>

  <p>
    <%= form.label :name %><br>
    <%= form.text_area :name %>
  </p>

  <p>
    <%= form.submit %>
  </p>
<% end %>

http://api.rubyonrails.org/v5.1/classes/ActionView/Helpers/FormOptionsHelper.html#method-i-select

logic for parks/:id/rangers

Because both routes point to a single controller we need to write some logic to get this to work- rails doesn't do that for us.

def index
  # test to see if we are at /parks/:id/rangers or /rangers
  if params.has_key?(:park_id)
    # get all the rangers for a specific park
    @rangers = Ranger.where(park_id: params[:park_id] )
  else
    # get all rangers
    @rangers = Ranger.all
  end
end

Exercise:

Repeat the steps above.

Further:

Implement the display logic for the nested new route: /parks/:id/rangers/new and the nested show route /parks/:id/rangers/:id

Further:

Implement the display logic for the nested index route: /parks/:id/rangers

Further:

Implement edit for the nested route