Magento Overriding Model

As part of a wider project, I am trying to overwrite the core class Mage_Sales_Model_Order_Payment. I am editing a config.xml file that is part of an already active module.

<config>
  <global>
    <models>
       <sales>
         <rewrite>
    <order_payment>MyNamespace_MyModule_Model_Payment</order_payment>
     </rewrite>
   </sales>
    </models>
  </global>
 <config>

The class I am trying to replace is in the directory:

app/code/local/MyNamespace/MyModule/Model/Pay开发者_C百科ment.php

It is called MyNamespace_Sales_Model_Order_Payment.

Currently, no overwriting seems to take place and the core class functions seem to be used. Any help would be very useful.

Thank you.

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The name of the class must match the file it's in. In this case it should be called MyNamespace_MyModule_Model_Payment