Using HABTM with a join table.

_staff_form.html.erb

<%# This will show a list of check_boxes for each service, if the staff has already enabled a service then it will show as checked %>
<%= f.collection_check_boxes(:services_ids, Service.all, :id, :to_s) %>

add_staffs_services_join_table.rb

# Use rails g migration add_staffs_services_join_table to create this file with the correct filename
class AddStaffsServicesJoinTable < ActiveRecord::Migration
  def change
    create_join_table :staffs, :services do |t|
      t.index :staff_id
      t.index :service_id
    end
  end
end

service.rb

class Service < ActiveRecord::Base
  has_and_belongs_to_many :staffs, join_table: 'staffs_services'
end

staff.rb

class Staff < ActiveRecord::Base
  has_and_belongs_to_many :services, join_table: 'staffs_services'
end

staffs_controller.rb

class StaffsController < ApplicationController
  def show
    @services = @staff.services
  end
  
    # Remember to add whitelist the service_ids param in the controller if using Rails 4. 
    # You'll need to set it to default to an array as it's a join_table attribute.
    # This is only needed if you're going to actually edit the association in the form,
    # i'm not sure if you want to be able to do that on both staff and services or not.
    def staff_params
      params.require(:staff).permit(service_ids: [])
    end
end

Ok cheers dude