How do I access a query string parameter in loopback model?

My api endpoint is like as follows for a model definition product.js

api/products/9720?id_shop=1&id_lang=1

I need to access the id_shop in product.js to apply a where clause before it fetches the records from products table.

Product.observe('access', function (ctx, next) {
    next();
});

How would I access id_shop and id_lang?