Rails 5 how to edit join model attributes from the parent model's view? - ruby-on-rails-5

Question

I have a system where admins can create Exams and price them according to the Branch they are selling them in. So for example, Exam One can cost $5 in Branch One, but $10 in Branch Two.
I made a join table named ExamOffering that has the price of the exam, so each Exam can be sold at many Branches at different prices, and each Branch can have many Exams. Like this:
class Branch < ApplicationRecord
has_many :exam_offerings
has_many :exams, through: :exam_offerings
end
class Exam < ApplicationRecord
has_many :exam_offerings
has_many :branches, through: :exam_offerings
end
class ExamOffering < ApplicationRecord
# this class has a 'price' attribute
belongs_to :exam
belongs_to :branch
end
I need to be able to create a new Exam and select a Branch in the form in order to put in a price, but that attribute is not part of the Exam model, but the ExamOffering join table. I've tried some ways but failed (using accepts_nested_attributes_for :exam_offerings in the Exam model or iterating through all the Branches and creating ExamOfferings for each one in the controller). What's the 'Rails way' of doing this? I think it's a pretty common case, but I haven't found an answer that fits my case. Maybe this has a name and I don't know it.
It could be phrased like this: When I'm creating a new Exam, I want to be able to input a price for each existing Branch.
Thanks.

Answer 1

What you want to do is create the join model explicitly:
# config/routes.rb
resources :exam_offerings, only: [:new, :create]
class ExamOffering < ApplicationRecord
# this class has a 'price' attribute
belongs_to :exam
belongs_to :branch
accepts_nested_attributes_for :exam
validates_associated :exam
end
# app/views/exam_offerings/new.html.erb
<%= form_with(model: #exam_offering) do |f| %>
<div class="field" %>
<%= f.label(:branch_id) %>
<%= f.collection_select(:branch_id, Branch.all, :id, :name) %>
</div>
<div class="field" %>
<%= f.label(:price) %>
<%= f.number_field(:price) %>
</div>
<%= f.fields_for(:exam) do |exam_field| %>
<div class="field" %>
<%= exam_field.label(:foo) %>
<%= exam_field.text_field(:foo) %>
</div>
<% end %>
<%= f.submit %>
<% end %>
class ExamOfferingsController < ApplicationController
# GET /exam_offerings/new
def new
#exam_offering = ExamOffering.new
end
# POST /exam_offerings
def create
#exam_offering = ExamOffering.new(exam_offering_params)
if #exam_offering.save
redirect_to #exam_offering, notice: 'Offering Created'
else
render :new, notice: 'Oh No!'
end
end
private
def exam_offering_params
params.require(:exam_offering)
.permit(
:branch_id, :price,
exam_attributes: [:foo]
)
end
end
Remember that there is nothing special about join models - and they don't always have to be created implicitly. Quite often they are actually an important part of the buisness logic and not just plumbing.
But really if I was building this I would just create a normal POST /exams route where the user can create an exam and then let them create the offering after that. Use AJAX if needed to make it appear seamless.

Answer 2

I ended up using accepts_nested_attributes_for in the Exam model, so I didn't have to use another view to put the price, so it's something like this:
class Branch < ApplicationRecord
has_many :exam_offerings
has_many :exams, through: :exam_offerings
end
class Exam < ApplicationRecord
has_many :exam_offerings
has_many :branches, through: :exam_offerings
accepts_nested_attributes_for :exam_offerings
end
class ExamOffering < ApplicationRecord
# this class has a 'price' attribute
belongs_to :exam
belongs_to :branch
end
The important part was the view. I needed to use fields_for with a new object when creating the exam, but fetch the existing exam_offerings when editing the exam, so I have this code:
<% if exam.exam_offerings.empty? %>
<% Branch.all.each do |branch| %>
<%= form.fields_for :exam_offerings, #exam.exam_offerings.build do |offering| %>
<div class="field">
<%= offering.label "Price in #{branch.name}" %>
<%= offering.text_field :price %>
<%= offering.hidden_field :branch_id, value: branch.id %>
</div>
<% end %>
<% end %>
<% else %>
<%= form.fields_for :exam_offerings do |offering| %>
<div class="field">
<%= offering.label "Price in #{offering.object.branch.name}" %>
<%= offering.text_field :price %>
</div>
<% end %>
<% end %>