* getByIds gets an array of objects as found with the given array of primary Keys.
* If no object is found, an empty returned.
* If $primaryKeys is not an array, it is expected to be a simple $primaryKey,
* so Persistence::getById() is called directly and the result returned in a
* single- element array.
* If the third $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.
// one line to get an array of complete objects...
$persistence = new Persistence();
$shirts = $persistence->getByIds("Shirt", array(10, 4, "test"), false); //, $isComplete = false