Question

Understanding Magento Models by reference of SQL:

1. select * from user_devices where user_id = 1
2. select * from user_devices where device_id = 3

How could I perform the same using my magento models? getModel("module/userdevice")

Also, how can I find the number of rows for each query

Following questions have been answered in this thread.

How to perform a where clause ?
How to retrieve the size of the result set ?
How to retrieve the first item in the result set ?
How to paginate the result set ? (limit)
How to name the model ?

Answer 1

You are referring to Collections

Some references for you:

1. http://www.magentocommerce.com/knowledge-base/entry/magento-for-dev-part-5-magento-models-and-orm-basics
2. http://alanstorm.com/magento_collections
3. http://www.magentocommerce.com/wiki/1_-_installation_and_configuration/using_collections_in_magento
4. lib/varien/data/collection/db.php and lib/varien/data/collection.php

So, assuming your module is set up correctly, you would use a collection to retrieve multiple objects of your model type.

Syntax for this is:

$yourCollection = Mage::getModel('module/userdevice')->getCollection()

Magento has provided some great features for developers to use with collections. So your example above is very simple to achieve:

$yourCollection = Mage::getModel('module/userdevice')->getCollection()
    ->addFieldToFilter('user_id', 1)
    ->addFieldToFilter('device_id', 3);

You can get the number of objects returned:

$yourCollection->count() or simply count($yourCollection)

EDIT

To answer the question posed in the comment: “what If I do not require a collection but rather just a particular object“

This depends if you still require both conditions in the original question to be satisfied or if you know the id of the object you wish to load.

If you know the id of the object then simply:

Mage::getModel('module/userdevice')->load($objectId);

but if you wish to still load based on the two attributes:

user_id = 1
device_id = 3

then you would still use a collection but simply return the first object (assuming that only one object could only ever satisfy both conditions).

For reuse, wrap this logic in a method and place in your model:

public function loadByUserDevice($userId, $deviceId)
{
    $collection = $this->getResourceCollection()
        ->addFieldToFilter('user_id', $userId)
        ->addFieldToFilter('device_id', $deviceId)
        ->setCurPage(1)
        ->setPageSize(1)
    ;

    foreach ($collection as $obj) {
        return $obj;
    }
    return false;
}

You would call this as follows:

$userId = 1;
$deviceId = 3;
Mage::getModel('module/userdevice')->loadByUserDevice($userId, $deviceId);

NOTE:

You could shorten the loadByUserDevice to the following, though you would not get the benefit of the false return value should no object be found:

public function loadByUserDevice($userId, $deviceId)
{
    $collection = $this->getResourceCollection()
        ->addFieldToFilter('user_id', $userId)
        ->addFieldToFilter('device_id', $deviceId)
    ;

    return $collection->getFirstItem();
}