Ordering by a related model attribute in Ruby/Sinatra

Question

I have simple Booking and Client models:

class Client
  include DataMapper::Resource
  property :id, Serial
  property :name, String, required: true
  property :email, String, required: true
  #....
  has n, :bookings, constraint: :destroy
end

class Booking  
  include DataMapper::Resource  
  property :id, Serial
  property :start_date, Date
  property :end_date, Date
  #....
  belongs_to :client
end

In the view a table is generated as expected:

<% @bookings.each do |booking| %>
  <tr>
    <td><%= booking.client.name %></td>
    <td><%= booking.start_date %></td>
    <td><%= booking.end_date %></td>
    ....
  </tr>
<% end %>

And I want order the table as by clicking on the respective table column heading.

I can get the table to sort by :start_date, or :end_date, and also by :client_id, but :client_id isn't very useful; I want to be able to sort by the :name attribute of the Client model with :client_id

Could someone tell me what format my controller needs to take, please?

get '/bookings' do
  @bookings = Booking.all(order: :client_id.name) #no method error
  @bookings = Booking.all(order: [:client_id][:name]) #can't convert to integer
  erb :bookings
end

Thanks