create_table :channels_displays, :id => false do |t|
  t.integer :display_id
  t.integer :channel_id
  t.integer :position
end


class Channel < OrganisationData
  has_many :displays, :through => :channel_display
end

# this join table has attribute "position"
class ChannelsDisplay < ActiveRecord::Base
  belongs_to :channel
  belongs_to :display
end

class Display < OrganisationData
  has_many :channels_display
  has_many :channels, :through => :channels_display, :order => :position
end

# but how set the position?
# display = Display.first
# display.channels << Channel.first
# ?