Magento collection classes confusion

Question

I confused by following collection classes

1. $orders = Mage::getResourceModel('sales/order_item_collection');

2. $orders = Mage::getResourceModel('sales/order');

3. $orders = Mage::getResourceModel('sales/order_collection');

   Whats the difference among those classes.

And whats difference between

Mage::getModel('sales/order');

and

 Mage::getResourceModel('sales/order');

Answer 1

1. Mage::getResourceModel('sales/order_item_collection') = Mage_Sales_Model_Resource_Order_Item_Collection this is the resource for a collection of items in orders, normally the database table that this is attached to is sales_flat_order_item
2. Mage::getResourceModel('sales/order') = Mage_Sales_Model_Resource_Order is the resource for a single sales orders and works with table sales_flat_order
3. Mage::getResourceModel('sales/order_collection') = Mage_Sales_Model_Resource_Order_Collection and this is the resource for the collection of orders and uses the table sales_flat_order.

The main difference between models, resources and collections is described as follows:

1. Model: these classes are used to representing a single item normally from the database,
2. Resource: these classes are used to fetch and save data for an item to the database,
3. Collection: these classes are used to deal with more than one item from the database,

Answer 2

They are all get different objects. They are all different files. They all serve different purposes. See here on how to use collections: https://www.magentocommerce.com/wiki/1_-_installation_and_configuration/using_collections_in_magento See here for Magento Models and ORM Basics: https://www.magentocommerce.com/knowledge-base/entry/magento-for-dev-part-5-magento-models-and-orm-basics