The list of files required for the Overriding model through a new module:
app/etc/modules/Magemonkeys_Catalog.xml
app/code/local/Magemonkeys/Catalog/etc/config.xml
app/code/local/Magemonkeys/Catalog/Model/Category.php

Creating Files and Folders: Custom Module
What we need to do is to create a module file.
Make a file – “app/etc/modules/Magemonkeys_Catalog.xml”
Then, add the below code inside the file

<?xml version="1.0"?>
<config>
  <modules>
    <Magemonkeys_Catalog>
      <active>true</active>
      <codePool>local</codePool>
    </Magemonkeys_Catalog>
  </modules>
</config>

Now create “app/code/local/Magemonkeys/Catalog/etc/config.xml” and add the below code inside the file:

<?xml version="1.0"?>
<config>
  <modules>
    <Magemonkeys_Catalog>
      <version>1.0</version>
    </Magemonkeys_Catalog>
  </modules>
   
  <global>
    <models>
      <catalog>
        <rewrite>
          <category>Magemonkeys_Catalog_Model_Category</category>
        </rewrite>
      </catalog>
    </models>
  </global>
</config>

We have defined a module version using the <version> tag. After that, the <catalog> & <rewrite> tags are used to override a “model” of the “Catalog” core module.

The final step will be to define a model class Magemonkeys_Catalog_Model_Category.
Make a model file “app/code/local/Magemonkeys/Catalog/Model/Category.php” and paste the following code inside it:

<?php
class Magemonkeys_Catalog_Model_Category extends Mage_Catalog_Model_Category
{
  public function getProductCollection()
  {
    // Put your custom code here!
 
    $collection = Mage::getResourceModel('catalog/product_collection')
      ->setStoreId($this->getStoreId())
      ->addCategoryFilter($this);
 
    return $collection;
  }
}
?>

That’s it.