Object property saving via $object->save(); problems - php

Question

I'm a magento programmer and I've been loosing several long minutes to figure out why a property on an object was not saved in the database.
Let's explain, here are 3 pieces of code that I would expect to do the same thing :
First code
$order = Mage::getModel('sales/order')->load(1873);
$myInfo = 'important piece of information';
$order->getPayment()->setAdditionalInformation('my_info',$myInfo);
$order->getPayment()->save(); //No information in the database is saved
No value saved in database.
Second code
$order = Mage::getModel('sales/order')->load(1873);
$myInfo = 'important piece of information';
$payment = $order->getPayment();
$payment->setAdditionalInformation('my_info',$myInfo);
$payment->save(); //No information in the database is saved
No value saved in database.
Third code
$order = Mage::getModel('sales/order')->load(1873);
$myInfo = 'important piece of information';
$order->getPayment()->setAdditionalInformation('my_info',$myInfo)->save(); //YEAHHH ! It works ! I now have that in my database.
Finally, I got it !
The code from setAdditionalInformation
/**
* Additional information setter
* Updates data inside the 'additional_information' array
* or all 'additional_information' if key is data array
*
* #param string|array $key
* #param mixed $value
* #return Mage_Payment_Model_Info
* #throws Mage_Core_Exception
*/
public function setAdditionalInformation($key, $value = null)
{
if (is_object($value)) {
Mage::throwException(Mage::helper('sales')->__('Payment disallow storing objects.'));
}
$this->_initAdditionalInformation();
if (is_array($key) && is_null($value)) {
$this->_additionalInformation = $key;
} else {
$this->_additionalInformation[$key] = $value;
}
return $this->setData('additional_information', $this->_additionalInformation);
}
note: The final setData() always returns $this
Question, Why ?
I think I've forgot some specificities about the way PHP works, especially for the first code. I would understand that it doesn't work because of some memory stuff with PHP.
But the two other pieces of code, why doesn't it work ?
Thanks,
Hugues.

Answer 1

These pieces of code are identical from Magento view - you didn't forget anything about how PHP works. With default Magento installation all 3 snippets must produce same results.
If the results of those code blocks are different, then you should:
a) turn off all custom extensions you use and try your code blocks without them - maybe some of extensions modify the default behavior of Order or Payment models.
b) check that your code snippets are really same as presented in this question - maybe there were other code lines that you thought of as non-important and didn't include in this question
c) check that you update view in your MySQL client after executing each code snippet - maybe you see just some old information in payment table
c2) check that you don't use replicated MySQL severs - maybe you update information on master DB, but sees payment table from slave DB, where these changes haven't yet been synced to
d) check that no other code executes after yours - maybe some other model or controller modifies additional_information and so deletes all your changes. Try to insert 'exit' just after your code so you'll be sure about it.

Answer 2

Not a Magento user, but it looks as if each method is returning an object which is required by the next method in the chain.
If you call each method individually, the object they create or modify won't contain any changes made by the previous method calls. By chaining the method calls, each one picks up the changes made by the previous call.