I'm trying to make a messaging system (sent messages, received messages, senders, receivers) with `Message` and `User`. Since it's a many to many relationship, I have a join model called `messages_users`

schema:

```
ActiveRecord::Schema.define(:version => 20121204230411) do

  create_table "messageattachments", :force => true do |t|
    t.string   "picture"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  create_table "messages", :force => true do |t|
    t.string   "content"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  create_table "messages_users", :force => true do |t|
    t.integer  "receiver_id"
    t.integer  "sender_id"
    t.datetime "created_at",          :null => false
    t.datetime "updated_at",          :null => false
    t.integer  "sent_message_id"
    t.integer  "received_message_id"
  end

  create_table "users", :force => true do |t|
    t.string   "name"
    t.string   "email"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

end

```

models:

```
class Message < ActiveRecord::Base
  attr_accessible :content

  has_many :messages_users
  has_many :receivers, :through => :messages_users
  has_many :senders, :through => :messages_users

end

class User < ActiveRecord::Base
  attr_accessible :email, :name

  has_many :messages_users
  has_many :sent_messages, :through => :messages_users
  has_many :received_messages, :through => :messages_users
end

class MessagesUsers < ActiveRecord::Base
  attr_accessible :sent_messages_id, :received_messages_id, :receiver_id, :sender_id

  belongs_to :sent_messages, :class_name => "Message", :foreign_key => "sent_message_id"
  belongs_to :received_messages, :class_name => "Message", :foreign_key => "received_message_id"
  belongs_to :receivers, :class_name => "User", :foreign_key => "receiver_id"
  belongs_to :senders, :class_name => "User", :foreign_key => "sender_id"
end

```

I'm trying to run a simple test:

```
require 'test_helper'

class MessageTest < ActiveSupport::TestCase
  test "has senders" do
    message = Message.create(:content => "hi")
    message.sender = User.first #Am I allowed to an instance of another Model in unit tests?
    assert message.sender.valid?
  end

```

regardless of what I test I always get this errror:

```
# Running tests:

E

Finished tests in 0.083961s, 11.9103 tests/s, 0.0000 assertions/s.

  1) Error:
test_has_senders(MessageTest):
ActiveRecord::StatementInvalid: PG::Error: ERROR:  column "receiver_id" of relation "messages" does not exist
LINE 1: INSERT INTO "messages" ("content", "receiver_id", "sender_id...
end

```

Can someone help me out? I don't see any issues with my models..

```
Loading development environment (Rails 3.2.8)
1.9.3-p194 :001 > message = Message.create(:content => "hi")
   (0.2ms)  BEGIN
  SQL (22.6ms)  INSERT INTO "messages" ("content", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["content", "hi"], ["created_at", Tue, 04 Dec 2012 23:58:19 UTC +00:00], ["updated_at", Tue, 04 Dec 2012 23:58:19 UTC +00:00]]
   (0.8ms)  COMMIT
 => #<Message id: 1, content: "hi", created_at: "2012-12-04 23:58:19", updated_at: "2012-12-04 23:58:19"> 
1.9.3-p194 :002 > message.sender = User.first
  User Load (0.6ms)  SELECT "users".* FROM "users" LIMIT 1
NoMethodError: undefined method `sender=' for #<Message:0x007f92441ac538>
    from /Users/edmundmai/.rvm/gems/ruby-1.9.3-p194@rails3tutorial2ndEd/gems/activemodel-3.2.8/lib/active_model/attribute_methods.rb:407:in `method_missing'
    from /Users/edmundmai/.rvm/gems/ruby-1.9.3-p194@rails3tutorial2ndEd/gems/activerecord-3.2.8/lib/active_record/attribute_methods.rb:149:in `method_missing'
    from (irb):2
    from /Users/edmundmai/.rvm/gems/ruby-1.9.3-p194@rails3tutorial2ndEd/gems/railties-3.2.8/lib/rails/commands/console.rb:47:in `start'
    from /Users/edmundmai/.rvm/gems/ruby-1.9.3-p194@rails3tutorial2ndEd/gems/railties-3.2.8/lib/rails/commands/console.rb:8:in `start'
    from /Users/edmundmai/.rvm/gems/ruby-1.9.3-p194@rails3tutorial2ndEd/gems/railties-3.2.8/lib/rails/commands.rb:41:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'
1.9.3-p194 :003 > 

```