class Project < ActiveRecord::Base  
　　has_many :tasks  
end

class Task < ActiveRecord::Base  
　　belongs_to :project  
end

で関連性をつけておくと

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

を

@project = Project.find(params[:id])  
@tasks = @project.tasks.find_all_by_complete(false)

に変えられコードを短くできるというお話。