cable.rb

class Cable < ActiveRecord::Base
  has_many :connections, :class_name => 'Port', :autosave => false
end

port.rb

class Port < ActiveRecord::Base
  belongs_to :cable, :dependent => :destroy
  has_many :connections, :through => :cable, :conditions => 'ports.id != #{self.id}'
end

#>> Port.last.connections.empty?
#ActiveRecord::StatementInvalid: Mysql::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right
#syntax to use near ') AND ((ports.id != 1768)))' at line 1: SELECT count(*) AS count_all FROM `ports`  INNER JOIN `cables` ON `ports`.cable_id = `cables`.id
#WHERE ((`cables`.id = ) AND ((ports.id != 1768)))