rspec returns nil find_all_by

The statement
products = find_all_by_category_id(category_id)
in the Product model is returning nil with rspec.
I am expecting it to to return the stub that I provided it with. It does
not also work with fixtures.

Product model

def self.find_all_meeting_some_criteria_for_category(category_id)
    products = find_all_by_category_id(category_id)
    products.each do |product|

Product spec

it "should find products given a category" do
   product = mock_model(Product, :id => 1, :category_id => 1)