How to get a product 's reduction percentage / amount using it's id?

[prak]

Hello, I'm trying to create a module which displays chosen products under their category's name on the home page.

 

I am stuck at getting the reduction percentage / amount due to a catalog price rule targeting the product's default category. I'm getting the reduction due to specific price rules using this

 

 SpecificPrice::getByProductId($tshirts[1]->id);

 

I've tried using this for getting the reduction due to a catalog price rule:

 

 GroupReduction::getGroupsReductionByCategoryId($tshirts[1]->id_category_default);

 

this has not worked. It returns an empty array even though a catalog price rule is targeting that category.

 

Help would be greatly appreciated!