old test.rb

b = Book.get(1)

b.latest_page #=> yields Page with id 3, as if I did Page.first(:order => [ :book_id.desc ])

#if I replace the line in latest_page with
self.pages.first(:book_id => self.id, :order => [ :created_at.desc ])

b.latest_page #=> yields Page with id 2, as expected

#if I replace the line in latest_page with
self.pages.first

#this one is with big assumption that without ORDER BY in SELECT statement the DB returns
#the last created/modified entry (which is true for Postgres -- what I use in this case)
#and that page with id 3 was created after page with id 2 was created after page with id 1

b.latest_page #=> returns Page with id 2, as expected with the above assumption

spec.rb

require 'rubygems'
require 'dm-core'
require 'spec'

class Book
  include DataMapper::Resource
 
  property :id, Serial
  property :title, String
 
  has n, :pages
 
  def latest_page
    self.pages.first(:order => [ :created_at.desc ])
  end
end
 
class Page
  include DataMapper::Resource
 
  property :id, Serial
  property :book_id, Integer
  property :created_at, Date
 
  belongs_to :book
end

describe "association scoping problem?" do
  before(:each) do
    DataMapper.setup(:default, "sqlite3::memory:")
    
    Book.auto_migrate!
    Page.auto_migrate!

    @book_1 = Book.create(:title => "War and Peace")
    @page_1_1 = Page.create(:book_id => @book_1.id, :created_at => "2009-01-01")
    @page_1_2 = Page.create(:book_id => @book_1.id, :created_at => "2009-01-02")

    @book_2 = Book.create(:title => "The History of the Decline and Fall of the Roman Empire")
    @page_2_1 = Page.create(:book_id => @book_2.id, :created_at => "2009-02-01")
  end
  
  it "should return same as expected for book 1" do
    lookup = @book_1.pages.first(:order => [ :created_at.asc ])
    expected = @page_1_1
    
    puts lookup.inspect
    puts expected.inspect
    
    lookup.should == expected
  end
  
  it "should return same as expected for book 1" do
    lookup = @book_1.latest_page
    expected = @page_1_2
    
    puts lookup.inspect
    puts expected.inspect
    
    lookup.should == expected
  end
  
  it "should return same as explicitly scoped lookup for book 1" do
    lookup = @book_1.latest_page
    expected = @page_1_2
    
    puts lookup.inspect
    puts expected.inspect
    
    lookup.should == expected
  end
  
  it "should return same as expected for book 2" do
    lookup = @book_2.latest_page
    expected = @page_2_1
    
    puts lookup.inspect
    puts expected.inspect
    
    lookup.should == expected
  end
end