SO Answer

sections_controller.rb

class SectionsController < ApplicationController
  before_action :set_section, only: [:show, :edit, :update, :delete, :destroy]

  layout 'admin'

  def index
    @sections = Section.sorted
  end

  def show
  end

  def new
    @section = Section.new(:name => 'Default')
    set_pages
  end

  def create
    @section = Section.new(section_params)
    if @section.save
      flash[:notice] = "Section created successfully."
      redirect_to :action => 'index'
    else
      set_pages
      render 'new'
    end
  end

  def edit
    set_pages
  end

  def update
    if @section.update_attributes(section_params)
      flash[:notice] = "Section updated successfully."
      redirect_to :action => 'show', :id => @section.id
    else
      set_pages
      render 'edit'
    end
  end

  def delete
  end

  def destroy
    @section.destroy
    flash[:notice] = "Section destroyed successfully."
    redirect_to :action => 'index'
  end


  private
    def set_section
      @section = Section.find(params[:id])
    end

    def set_pages
      @pages = Page.order('position ASC')
      @section_count = Section.count + (@section.new_record? ? 1 : 0)
    end

    def section_params
      params.require(:section).permit(:name, :position, :content)
    end
end

So, how do I nest Sections into the Page?
my Sections_Controller looks exactly like this. But it gives me Undefined error while I tried to nest Sections into Page.