Rails: Multi-level nested form (has_many association with checkboxes)

assign_vendor.rb

# Bridge table between vendors & users
class AssignVendor < ActiveRecord::Base
  belongs_to :vendor 
  belongs_to :user
  has_many :assign_vendor_docs
  has_many :vendor_files, through: :assign_vendor_docs
  
  accepts_nested_attributes_for :assign_vendor_docs, allow_destroy: true
end

assign_vendor_doc.rb

# Bridge table between vendor_files & assign_vendors(also a bridge table)
class AssignVendorDoc < ActiveRecord::Base
  belongs_to :vendor_file
  belongs_to :assign_vendor
end

nested_form.html.erb

<%= simple_form_for [@new_user] do |f| %>
  <%= f.input :first_name, input_html: {required: true} %>
  <%= f.input :email, input_html: {required: true} %>
  <% Vendor.all.each_with_index do |vendor, index| %>
    <%= check_box_tag "user[assign_vendors_attributes][#{index}][vendor_id]", vendor.id, @user.vendors.include?(vendor), id: dom_id(vendor) %>
    <%= label_tag dom_id(vendor), vendor.name %>
    <% vendor.vendor_files.each_with_index do |file, f_index| %>
      <%= check_box_tag "users[assign_vendors_attributes][#{index}][assign_vendor_docs_attributes][#{f_index}][vendor_file_id]", file.id, false, id: dom_id(file) %>
      <%= label_tag dom_id(file), file.filename %>
    <% end %>
  <% end %>
<% end %>

user.rb

class User < ActiveRecord::Base
  has_many :assign_vendors
  has_many :vendors, through: :assign_vendors
  
  accepts_nested_attributes_for :assign_vendors, allow_destroy: true
end

users_controller.rb

class UserController < ApplicationController
  
  def new
    @new_user = User.new
    @new_user.assign_vendors.build
    respond_with(@new_user)
  end
  
  def create
    @user = User.new(user_params)
    if @user.save
      flash[:notice] = 'User successfully created.'
    end
    redirect_to #anywhere you want
  end

  def user_params
    params.require(:user).permit(:first_name, :last_name, :email,
      assign_vendors_attributes: [:vendor_id, :_delete, assign_vendor_docs_attributes: [:vendor_file_id, :_delete]])
  end
end

vendor.rb

class Vendor < ActiveRecord::Base
  has_many :vendor_files, dependent: :destroy
  has_many :assign_vendors
  has_many :users, through: :assign_vendors

  accepts_nested_attributes_for :vendor_files, allow_destroy: true
end

vendor_file.rb

class VendorFile < ActiveRecord::Base
  belongs_to :vendor

  has_many :assign_vendor_docs
  has_many :assign_vendors, through: :assign_vendor_docs
end