On an edit page, attach additional fields_for to a nested objec

case_management.rb

class CaseManagement < ActiveRecord::Base

  has_one :service, as: :serviceable

  has_many :treatment_plan_problems, dependent: :destroy
  accepts_nested_attributes_for :treatment_plan_problems, allow_destroy: true

  validates_associated :treatment_plan_problems

end

case_managements_controller.rb

class CaseManagementsController < ApplicationController

  def edit
    @case_management = CaseManagement.find(params[:id])
  end

  def update
    @case_management = CaseManagement.find(params[:id])
    if @case_management.update_attributes(case_management_params)
      redirect_to @case_management, notice: "Successfully updated case management."
    else
      render :edit
    end
  end

  def show
    @case_management = CaseManagement.find(params[:id])
  end

  def case_management_params
    params.require(:case_management).permit(:service_id, :new_problem, :counselor_id, :student_id, :stage, treatment_plan_problems_attributes: [:id, :initial, :domain, :treatment_plan_id, :problem_id, :problem_identifier, :problem_status, :problem_resolved, :client_statement, :severity, :severity_change, :current_severity, :_destroy, treatment_plan_problem_plan_attributes: [:treatment_plan_problem_id, :status, :resolved, :resolved_methods, :resolved_triggers, :continue, :prevented, :barriers, :this_time, :differently, :goal_change, :new_problem, :_destroy]])
  end
end

edit.html.erb

<%= simple_form_for @case_management, html: {class: 'form-horizontal'} do |f| %>

        <%= f.simple_fields_for :treatment_plan_problems do |problem| %>

              <h4><%= Problem.find(problem.object.problem_identifier).item %></h4>
              <hr>
              <h5><%= problem.object.domain %></h5>
              <table>
                <tbody>
                <tr>
                  <td>Client's statement of the problem</td>
                  <td><%= problem.object.client_statement %></td>
                </tr>
                <tr>
                  <td>Severity</td>
                  <td><%= problem.object.current_severity %></td>
                </tr>
                </tbody>
              </table>

       
            <%= f.simple_fields_for :treatment_plan_problem_plan do |plan| %>
            
                # THE FOLLOWING FIELD :status SHOULD BE ADDED TO A TREATMENT PLAN PROBLEM BUT DOES NOT SHOW UP IN PARAMS HASH. I feel like the above line should be problem.simple_fields_for instead of f.simple_fields_for, but then the following :status field does not display on the form.
                 
                <%= plan.input :status, label: 'What is the current status of the problem?', collection: ['Problem has been resolved', 'Problem still exists, continue with same plan', 'Problem still exists, change goals and methods'], input_html: {class: 'problem-plan-status'} %>
           
            <% end %>
            
<% end %>

treatment_plan_problem.rb

class TreatmentPlanProblem < ActiveRecord::Base

  belongs_to :case_management

  has_one :treatment_plan_problem_plan, dependent: :destroy
  accepts_nested_attributes_for :treatment_plan_problem_plan, allow_destroy: true

  validates_associated :treatment_plan_goals
  validates_associated :treatment_plan_problem_plan

end

treatment_plan_problem_plan.rb

class TreatmentPlanProblemPlan < ActiveRecord::Base

  belongs_to :treatment_plan_problem

  validates :status, presence: true

end