Ruby on Rails

The below query fails:

Timesheet.joins(:time_entries).select("timesheets.*, sum(time_entries.worktime) as total").group("timesheets.start_date")

The models have the following relations:

Timesheet < AR

  has_many :activities, dependent: :destroy, inverse_of: :timesheet

  has_many :time_entries, through: :activities

  accepts_nested_attributes_for :activities, allow_destroy: true

end

class Activity < AR

  belongs_to :timesheet, inverse_of: :activities

  belongs_to :task

  has_many :time_entries, order: :workdate, dependent: :destroy, inverse_of: :activity

  accepts_nested_attributes_for :time_entries, allow_destroy: true, reject_if: proc { |a| a[:worktime].blank? }

end

class TimeEntry < AR

  belongs_to :activity, :inverse_of => :time_entries

end

How is it possible to group all the time sheets by their start_date and sum the work time ?

Thank you.