## transaction.rb
class Transaction < ActiveRecord::Base
  has_one :seller_agent
  has_one :agent, :through => :seller_agent
end

## seller_agent.rb
class SellerAgent < ActiveRecord::Base
  belongs_to :agent
  belongs_to :transaction
end

## agent.rb
class Agent < ActiveRecord::Base
  has_many :seller_agents
  has_many :transactions, :through => :seller_agents
end

## summary of ./script/console output
Loading development environment (Rails 2.1.1)
>> tr = Transaction.new
=> #<Transaction id: nil, trans_number: nil, created_at: nil, updated_at: nil>
>> tr.trans_number = 2221
=> 2221
>> tr.save
=> true
>> tr.agent
>> ag = Agent.new
=> #<Agent id: nil, username: nil, created_at: nil, updated_at: nil>
>> ag.username = 'justme'
=> "justme"
>> ag.save
=> true
>> tr.agent = ag
=> #<Agent id: 1, username: "justme", created_at: "2008-10-15 01:47:36", updated_at: "2008-10-15 01:47:36">
>> tr.agent
=> nil
>> tr.save
=> true
>> tr.agent
=> nil
>> tr.seller_agent
=> nil
>> SellerAgent.find :all
=> []
>> sa = SellerAgent.new
=> #<SellerAgent id: nil, agent_id: nil, transaction_id: nil, created_at: nil, updated_at: nil>
>> sa.agent = ag
=> #<Agent id: 1, username: "justme", created_at: "2008-10-15 01:47:36", updated_at: "2008-10-15 01:47:36">
>> sa.transaction = tr
=> #<Transaction id: 1, trans_number: 2221, created_at: "2008-10-15 01:46:22", updated_at: "2008-10-15 01:46:22">
>> sa.transaction
=> #<Transaction id: 1, trans_number: 2221, created_at: "2008-10-15 01:46:22", updated_at: "2008-10-15 01:46:22">
>> sa.save
=> #<Transaction id: 1, trans_number: 2221, created_at: "2008-10-15 01:46:22", updated_at: "2008-10-15 01:46:22">
>> sa
=> #<SellerAgent id: nil, agent_id: 1, transaction_id: 1, created_at: nil, updated_at: nil>
>> sa.save
=> #<Transaction id: 1, trans_number: 2221, created_at: "2008-10-15 01:46:22", updated_at: "2008-10-15 01:46:22">
>> tr.seller_agent
=> nil
>> tr.agent
=> nil
>> tr = nil
=> nil
>> tr = Transaction.find :first
=> #<Transaction id: 1, trans_number: 2221, created_at: "2008-10-15 01:46:22", updated_at: "2008-10-15 01:46:22">
>> tr.agent
=> nil
>> tr.seller_agent
=> nil
>> sa
=> #<SellerAgent id: nil, agent_id: 1, transaction_id: 1, created_at: nil, updated_at: nil>
>> sa.transaction
=> #<Transaction id: 1, trans_number: 2221, created_at: "2008-10-15 01:46:22", updated_at: "2008-10-15 01:46:22">
>> sa.agent
=> #<Agent id: 1, username: "justme", created_at: "2008-10-15 01:47:36", updated_at: "2008-10-15 01:47:36">