* getById gets an object as found with the given primary Key.
* If no object is found, null is returned.
* If multiple objects are found (what should not occur if the param is a real primary key),
* only the first object is returned.
* If a third, optional, "$isComplete" param is set to false, only the main object is
* returned, not the agregated or linked subobjects.
* In this case the members variables holding the subobjects are set to the subobjects identifier.
* Inheritence is always used.
// Lets get the wine.
$persistence = new Persistence();
$wine = $persistence->getById("Wine", 1);
// Logger::debug prints a dump of the object