public function testDiffWithACollectionHavingObjectsNotPresentInTheFirstCollection()
{
$col1 = new PropelCollection();
$col2 = new PropelCollection();
$b = new Book();
$col2[] = $b;
$result = $col1->diff($col2);
$this->assertInstanceOf('PropelCollection', $result);
$this->assertEquals(0, count($result));
}
