Using ActiveRecord.create with belongs_to associations

I'm using Rails 3.1 and have a simple transaction class that references a User model:

class User < ActiveRecord::Base has_many :transactions, :foreign_key => 'sender_id' end class Transaction < ActiveRecord::Base belongs_to :sender, :class_name => 'User' belongs_to :recipient, :class_name => 'User' attr_accessible :amount end 

The database schema is

create_table "transactions", :force => true do |t| t.decimal "amount" t.integer "sender_id", :null => false t.integer "recipient_id", :null => false end create_table "users", :force => true do |t| end 

I would like to create a set of initial transactions in the seeds.rb, but the underlying INSERT is never generated with foreign keys for the sender_id and recipient_id. The signature of the method is:

ruby-1.9.2-p290 :022 > Transaction.method(:create) => #<Method: Transaction(id: integer, amount: decimal, sender_id: integer, recipient_id: integer) (ActiveRecord::Base).create> 

I've tried both

Transaction.create( amount: 0.50, sender: User.first, recipient: User.last, ) 

and

Transaction.create( amount: 0.50, sender_id: User.first.id, recipient_id: User.last.id, ) 

In each case, the INSERT statement is

SQL (0.6ms) INSERT INTO `transactions` (`amount`, `recipient_id`, `sender_id`) VALUES (0.75, NULL, NULL) 

I'm new to rails, so I'm sure this is a misunderstanding on my part, but I have not been able to find a solution from my reading of the rails docs.