Some notes about relationships in Rails

I had been in troubles setting up some relationships… in Rails too. Just to avoid the same headhecks in the future I decided to write this post. This code is supposed to help me (and you!) to make things easier next time.

The code has been tested on ruby-2.3.0 and Rails 5.0.2.

One_to_many relationship

As an example, one Author can have many Interests.

	# app/models/author.rb
	class Author < ActiveRecord::Base
	has_many :interests
	end
	# app/models/interest.rb
	class Interest < ActiveRecord::Base
	belongs_to :author
	end
	# app/controllers/authors_controller.rb
	...
	private
	def set_author
	@author = Author.find(params[:id])
	end
	def author_params
	params.require(:author).permit(:name, :interest_id)
	end
	...
	# app/views/authors/_form.html.erb
	<%= form_for(@author) do |f| %>
	<% if @author.errors.any? %>
	<div id="error_explanation">
	<h2><%= pluralize(@author.errors.count, "error") %> prohibited this author from being saved:</h2>
	<ul>
	<% @author.errors.full_messages.each do |message| %>
	<li><%= message %></li>
	<% end %>
	</ul>
	</div>
	<% end %>
	<div class="field">
	<%= f.label :name %><br>
	<%= f.text_field :name %>
	</div>
	<div class="field">
	<%= f.label :author_id %><br>
	<%= f.collection_select(:interest_id, Interest.all, :id, :name, prompt: true) %>
	</div>
	<div class="actions">
	<%= f.submit %>
	</div>
	<% end %>

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Using has_and_belongs_to_many

	# using has_and_belongs_to_many
	rails g model mountain name
	rails g model climber name
	class Climber < ApplicationRecord
	has_and_belongs_to_many :mountains
	end
	class Mountain < ApplicationRecord
	has_and_belongs_to_many :climbers
	end
	# using a model...
	rails g model climbers_mountains climber:references mountain:references
	# ...Rails will create
	class CreateClimbersMountains < ActiveRecord::Migration[5.0]
	def change
	create_table :climbers_mountains do |t|
	t.references :climber, foreign_key: true
	t.references :mountain, foreign_key: true
	t.timestamps
	end
	end
	end
	# using a migration
	rails g migration CreateJoinTableClimbersMountains climber mountain
	# ...Rails will create this. You must uncomment one of the t.index lines
	class CreateJoinTableClimbersMountains < ActiveRecord::Migration[5.0]
	def change
	create_join_table :climbers, :mountains do |t|
	t.index [:climber_id, :mountain_id]
	# t.index [:mountain_id, :climber_id]
	end
	end
	end
	# then you should execute
	$ rake db:create
	$ rake db:migrate (or 'rake db:migrate RAILS_ENV=test' for the test environment)
	$ rails c
	john = Climber.create(name: "John Doe")
	udalaitz = Mountain.create(name: "Udalaitz")
	john.mountains << udalaitz
	===

view raw has_and_belongs_to_many.txt hosted with by GitHub

Using has_many through

I prefer to use the ‘has_many :through’ based solution. It is possible to create a Model associated to the relationship, with attributes, so the solution is much more flexible.

	# using has_many :through
	rails g model mountain name
	rails g model climber name
	rails g model adventure climber:references mountain:references
	class Climber < ApplicationRecord
	has_many :adventures
	has_many :mountains, :through => :adventures
	end
	class Mountain < ApplicationRecord
	has_many :adventures
	has_many :climbers, :through => :adventures
	end
	class Adventure < ApplicationRecord
	belongs_to :climber
	belongs_to :mountain
	end
	$ rake db:create
	$ rake db:migrate (or 'rake db:migrate RAILS_ENV=test' for the test environment)
	$ rails c
	john = Climber.create(name: "John Doe")
	udalaitz = Mountain.create(name: "Udalaitz")
	john.mountains << udalaitz

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