RailsCasts 003 find through association

关联关系：
Project has_many Tasks
Task belongs_to Project
传统做法：

@project = Project.find(params[:id])
@tasks = Task.find(:all, :conditions => ['project_id = ? AND complete = ?'], @project.id, false)

Rails做法：

@tasks = @project.tasks.find(:all, :conditions => ['complete = ?'], false)

用episode 002介绍的动态方法进一步简化：

@tasks = @project.tasks.find_all_by_complete(false)