Many to Many Associations

Objectives

Implement many to many relationships through models in Rails
Understand the model ordering opinion used by Rails
Use the collection_check_boxes form helper to display a collection of associated items

Create the app:

rails new parkng -d postgresql

What we need

Models
- Park
- Ranger
Association
- Park <-> ParksRangers <-> Ranger
- Park has_and_belongs_to_many Rangers
- Ranger has_and_belongs_to_many Parks
Views
- parks#new - add/remove rangers checkbox
- parks#show
  - list all rangers with a specific park
- rangers#new - add/remove rangers checkbox
- rangers#show
  - list all parks with a specific ranger
Controllers
- parks#new - get a list of all rangers here
- parks#show
  - list all rangers with a specific park
- rangers#new - get a list of all parks here
- rangers#show
  - list all parks with a specific ranger
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

Rangers

rails generate migration rangers

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

ParksRangers

rails generate migration parks_rangers

create_table :parks_rangers do |t|
  t.references :park
  t.references :ranger
  t.timestamps
end

IMPORTANT

The join table with the two models must be plural and in alphabetical order if you want to follow the Rails convention.

Note that if you want to name your join table something different, you can specify your own join model with through:

Setting up associations

When you do :references it automatically creates the belongs_to relations on the join table, but we need to manually add the has_and_belongs_to_many to the ranger and park models.

models/park.rb

has_and_belongs_to_many :rangers

models/ranger.rb

has_and_belongs_to_many :parks

Adding rangers on the command line:

Given:

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

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

Add some stuff to the park:

some_park = Park.first
some_ranger = Ranger.first

# adding a ranger
some_park.rangers << some_ranger

When creating a new park:

yosemite = Park.new(name: "yosemite", description: "very noice", ranger_ids: [1,2])

Or assuming you already have 2 ranger instances:

ecp = Park.new(name: "ecp", description: "so good", rangers: [roger, sam])

Removing rangers on the command line:

# assume the following:
some_park = Park.first
some_ranger = Ranger.first

# clear all of the park's rangers (leaves the rangers in the table)
some_park.rangers.clear

# removes a specific ranger from a park (leaves the ranger in the table)
some_park.rangers.delete(some_ranger)

# removes a specific ranger from a park (and deletes the ranger)
some_park.rangers.first.destroy

Referencing and listing on the command line

Because Park and Ranger reference each other with has_and_belongs_to_many they can reference each other.

Basic Examples

#lists all rangers
Ranger.all

#lists all parks
Park.all

#gets first park in the database
Park.first

#lists all rangers of first park
Park.first.rangers

#lists all parks of first ranger
Ranger.first.parks

Set up our requests:

Nested routes


  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'

  get '/rangers/:ranger_id/parks' => 'parks#index', as: 'ranger_parks'
  get '/rangers/:ranger_id/parks/new' => 'parks#create', as: 'new_ranger_parks'
  post '/rangers/:ranger_id/parks' => 'parks#create'

view helpers:

Prepare in the controller: make an instance variable with @rangers
```
@rangers = Ranger.all
```

Add checkbox to the form:

<%= f.collection_check_boxes :ranger_ids, @rangers, :id, :name %>

And to the new park form as well:

@parks = Park.all

<%= f.collection_check_boxes :ranger_ids, @rangers, :id, :name %>

:ranger_ids refers to the name of the input and the key it will be in the request params
@rangers refers to all the rangers available (pass from the controller Ranger.all)
:id refers to the value of the checkbox when submitted in the request
:name refers to the label of the checkbox

parks and rangers controllers:

Modify the Park controller to accept the ranger_ids and parks_ids array like so:

def park_params
  params.require(:park).permit(:name, :description, :ranger_ids => [])
end

def ranger_params
  params.require(:ranger).permit(:name, :park_ids => [])
end

rangers#show

When showing a specific ranger, display the ranger name and the list of parks associated with it.

Rails M:M