Laravel 5.1 eloquent belongsTo relationship joining on multiple columns

I am connecting to a remote database that has been designed poorly but I can't amend it in any way I only have read access to the get the data I need. It has the following structure:

Products
    - id
    - style_id
    - department_id

Brands
    - id
    - Name
    - style_id
    - department_id

So as you can see rather than a product just having a brand_id field it has a style_id and department_id that you have to join on in order to find out what brand a product is.

So how would I set up my belongsTo relationship in my Product model in order to achieve this?

via Chebli Mohamed