b

lets say your product have relation to model example.example which is named in your product model example_id then you need to check if the example_id is null or not by this:

@api.multi
def unlink(self):
    for prod in self:

        if not prod.example_id:

            super(ProductProduct, self).unlink()

but as far as i know product.product model is important model.. which is the original product.product model has many relations to another model..