multidimensional Array or Hash with group_by 'created_at'

Hello, i want to create a multidimensional Array or Hash with group_by 'created_at'.

3 dimensions needed: myArray[week][day][data]

currently i only created it with 2 dimesions:

#model def self.group_per_day   all.group_by{|t| t.created_at.strftime("%d. %B, %Y")}.sort #"%U" for Weeks end

#controller @pdf_activities = Activity.group_per_day

#view -@pdf_activities.each do |day|   =day[0]   -day[1].each do |activity|     =activity.activity

thanks