title: How to use filter in Model
body: <p>A model named Post, corresponding to posts table in the db. <code>Post.find_within_4_days</code> can get me the last 4 days posts. All my following operation would be based on the past 4 days posts. I would like it to be filtered or defined somewhere, so I can just reference it instead of repeating <code>Post.find_within_4_days</code> everywhere. How do I do in Post model?</p>