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If an action needs data to be changed, the Controller will "ask" the Model to change the data, and in turn the Model will notify the View that the data has been changed, so that the View can update itself.
Database schema
You can download the PrestaShop 1.5 6 SQL schema in PDF form (3.76 90 Mb), or in the original MySQL Workbench file format (you will need MySQL Workbench to view it).