1. A method for generating a recommendation, the method comprising: tracking, on a server including a processor, user activity associated with a user interacting with a network-based system; determining, on the server, a recommendation relationship type based at least in part on the tracked user activity, the recommendation relationship type selected from a group of recommendation relationship types including a substitute relationship type and a complement relationship type; selecting, on the server, a recommended object based at least in part on the recommendation relationship type and a first object accessed by the user interacting with the network-based system; and generating, on the server, the recommendation for the recommended object for presentation to the user on the network-based system.