_form.rb

.form-group
  = f.label "Parent", class: "col-lg-2 col-md-2 col-sm-12 control-label"
  .col-lg-10.col-md-10
    = f.select(:navigation_link_id, options_for_select((@top_level_links.collect { |p| [ p.name, p.id ] } << ["Top", nil]).reverse, selected: navigation_link.parent_id))

application_controller.rb

  def top_level_links
    @top_level_links ||= NavigationLink.all
  end

navigation_link.rb

class NavigationLink < ActiveRecord::Base
  belongs_to :navigation_link, :class_name => "NavigationLink"
  has_many :navigation_links, :foreign_key => "navigation_link_id"

  def has_children?
    !self.navigation_links.empty?
  end

  def is_not_child?
    self.navigation_link_id.nil?
  end

  def parent_id
     self.navigation_link_id if self.navigation_link
  end

  def self.top_level_links
    where("navigation_link_id IS ? ", nil)
  end
end

schema.rb

create_table "navigation_links", force: :cascade do |t|
    t.string   "path"
    t.string   "name"
    t.integer  "navigation_link_id"
    t.datetime "created_at",         null: false
    t.datetime "updated_at",         null: false
    t.boolean  "sublink"
    t.integer  "position"
    t.index ["navigation_link_id"], name: "index_navigation_links_on_navigation_link_id", using: :btree