Find which page an item is on

Here's how I did it once:

def self.page_for_pagination(id, per_page = 10)
  return if (id = id.to_i) < 1
  query = "SELECT id FROM %s ORDER BY id DESC" % table_name
  find_by_sql(query).map(&:id).index(id).to_i / per_page + 1
end

If we extract pagination logic, we could slide through a window of 200 rows or something trying to find the id...