Rails 4 nested attributes and has_many :through associaton in a form

Question

I am having an issue managing a has_many :through association using a form. What I DON'T want to do is edit the attributes of the associated model of which there is a plethora of information out there, rather, I want to manage the association ONLY. I understand I could do this by manipulating the form parameters passed to my action and build the relationships manually, but I would prefer to go the Rails way, if that is possible.

One interesting thing about my case is that this has_many :through association is actually on a Model which I am already saving using accepts_nested_attributes_for

Here are my models: Goals, Milestones and Programs.

class Goal < ActiveRecord::Base
  has_many :milestones, inverse_of: :goal, dependent: :destroy
  accepts_nested_attributes_for :milestones, :allow_destroy => true
end

class Milestone < ActiveRecord::Base
  belongs_to :goal, inverse_of: :milestones

  has_many :milestone_programs
  has_many :programs, :through => :milestone_programs
end

class Program < ActiveRecord::Base
end

Now in my Goal edit view, I need to be able to add and remove milestones, and for those milestones I need to be able to add and remove program associations. This is the code for my form.

<%= form_for @goal do |f| %>

  <%= f.fields_for :milestones do |f_milestone| %>

    <%= f.hidden_field :id, :value => f.object.id %>
    <%= f.hidden_field :name, :value => f.object.name %>
    <a href="javascript:void(0)" class="milestone-remove">- remove</a>

    <ul>
      <%= f.fields_for :programs do |f_prog| %>
        <li>
          <%= f_prog.object.name %>
          <a href="javascript:void(0)" class="program-remove">- remove</a>
        </li>
      <% end %>
    </ul>

  <% end %>

  <%= f.submit 'Save' %>

<% end %>

In my controller, I have

class GoalsController < ApplicationController

    # PATCH/PUT /goals/:id
    def update
      if @goal.update(goal_params)
        redirect_to @goal
      end
    end

    def goal_params
      params.require(:goal).permit(:name, :milestones_attributes => [ :id, :name, :_destroy ])
    end

end

This form needs to be like a worksheet where you can make changes and only save your changes once you click save at the end, so I don't believe gems such as cocoon or nested_forms will help.

My code works perfectly so far for managing my Goals associated Milestones and their attributes. But now I want to be able to manage the list of Programs associated to those Milestones.

I have tried using accepts_nested_attributes_for but that is not exactly what I want because I don't care to edit the nested attributes of the model, the Program attributes are to remain static.

I thought I might be able to have something like this in my form for each program to build the associations:

<input type="hidden" name="goal[milestones_attributes][1][program_ids][1]" >

But that doesn't work either (of course I've added :program_ids to my white-listed parameters). Is there a magic rails method I need to add to my controller?

What am I missing here?

Thanks in advance!

Answer 1

When employing a has_many :through relationship, you need to pass the nested_attributes through the different models, like this:

Models

class Goal < ActiveRecord::Base
  has_many :milestones, inverse_of: :goal, dependent: :destroy
  accepts_nested_attributes_for :milestones, :allow_destroy => true

  def self.build
      goal = self.new
      goal.milestones.build.milestone_programs.build_program
  end
end

class Milestone < ActiveRecord::Base
  belongs_to :goal, inverse_of: :milestones

  has_many :milestone_programs
  has_many :programs, through: :milestone_programs

  accepts_nested_attributes_for :milestone_programs
end

class MilestoneProgram < ActiveRecord::Base
    belongs_to :milestone
    belongs_to :program

    accepts_nested_attributes_for :program
end

class Program
    has_many :milestone_programs
    has_many :milestones, through: :milestone_programs
end

Controller

#app/controllers/goals_controller.rb
def new
    @goal = Goal.build
end

def create
    @goal = Goal.new(goal_params)
    @goal.save
end

private

def goal_params
    params.require(:goal).permit(milestones_attributes: [milestone_programs_attributes: [program_attributes:[]]])
end

Form

#app/views/goals/new.html.erb
<%= form_for @goal do |f| %>
   <%= f.fields_for :milestones do |m| %>
      <%= m.fields_for :milestone_programs do |mp| %>
          <%= mp.fields_for :program do |p| %>
               <%= p.text_field :name %>
          <% end %>
      <% end %>
   <% end %>
   <%= f.submit %>
<% end %>

I appreciate this might not be exactly what you're looking for, but tbh I didn't read all your prose. I just gathered you needed help passing nested_attributes through a has_many :through relationship