class Invoice < ActiveRecord::Base
  belongs_to :employee
end

class Employee < ActiveRecord::Base
  has_many :invoices

  # the select_menu scope takes an employee instance as an argument
  # and adds this employee to the list of 'active' employees
  scope :active, where(active: true)
  scope :select_menu, lambda { |another_employee| active << another_employee }
end

/views/invoices/_form.html.erb

<%= simple_form_for @invoice do |f| %>
  <%= f.association :employee, collection: Employee.select_menu(@invoice.employee).sort_by(&:name), label_method: :first_last, include_blank: "" %>
  ...
<% end %>