class Property < ActiveRecord::Base
  has_many :property_agents
  has_many :agents, :through => :property_agents
end

class Agent < ActiveRecord::Base
  has_many :property_agents
  has_many :properties, :through => :property_agents
end

class PropertyAgent < ActiveRecord::Base
  belongs_to :agent
  belongs_to :property
end


 property_agents table
   :agent_id
   :property_id
   :property_key  # the id of the property on the agent's system

  Of all the agents Property belongs to, *one* is the 'foo' agent 
  and one is the 'bar' agent. 

  These agents fulfill a slightly different role than the others. They 
  need to be referenced via the property_agents model as I need the 
  associated property_key.

  so has_one foo_agent, class_name => agents 

  wouldn't work well, as it forgoes the join.