gistfile1.rb

$ rails -v
Rails 2.3.5

$ rails bugtest
$ cd bugtest


$ script/generate model Project name:string
$ cat app/models/project.rb
class Project < ActiveRecord::Base
  has_and_belongs_to_many :users
end


$ script/generate model User name:string
$ cat app/models/user.rb
class User < ActiveRecord::Base
  has_and_belongs_to_many :projects
end


$ script/generate migration CreateProjectsUsers
class CreateProjectsUsers < ActiveRecord::Migration
  def self.up
    create_table :projects_users, :id => false do |t|
      t.integer :project_id
      t.integer :user_id
    end
  end

  def self.down
    drop_table :projects_users
  end
end


$ rake db:test:prepare

$ cat bug_test.rb
require "test/test_helper"

class BugTest < ActiveSupport::TestCase
  def test_bug
    rails = Project.create :name => "Rails"
    js = rails.users.build :name => "JS"
    js.save!
    assert_equal [js], rails.users
    assert_equal [rails], js.projects
  end
end

$ ruby bug_test.rb
Loaded suite bug_test
Started
F
Finished in 0.33 seconds.

  1) Failure:
test_bug(BugTest) [bug_test.rb:8]:
<[#<User id: 980190963, name: "JS", created_at: "2010-01-30 19:52:34", updated_at: "2010-01-30 19:52:34">]> ex
pected but was
<[]>.

1 tests, 1 assertions, 1 failures, 0 errors

gistfile2.txt