nested forms: use validate :some_method works in update mode only

[Serguei Cambour]

I have a strange behavior when validating a record in a nested form. The association collection is empty when creating a new record and the validation does not catch the error. But when updating the same record the validation gets it right but continues to catch the error even after modifying the needed attribute value to make it pass. 

Here is the model:

#timesheet.rb

class Timesheet < ActiveRecord::Base
  attr_accessible :status, :user_id, :start_date, :end_date, :activities_attributes
 
  SUBMITTED = 'Submitted'
  APPROUVED = 'Approuved'
  REJECTED = 'Rejected'
 
  STATUS_VALUES = [SUBMITTED, APPROUVED, REJECTED]
  belongs_to :user
  has_many :activities, dependent: :destroy, inverse_of: :timesheet
  has_many :time_entries, through: :activities
  accepts_nested_attributes_for :activities, allow_destroy: true
 
  validates :user_id, presence: true
  validates :status, presence: true, inclusion: {in: STATUS_VALUES} 
  validate :maximum_worktime_per_day
 
  after_update :check_an_activity_present
 
  after_initialize :init_working_week

..
private
  def maximum_worktime_per_day
      time_entries_by_date = time_entries.group_by(&:workdate)
      time_entries_by_date.each do |key, value|
        errors[:base] << "Maximum daily time should not exceed 1 day" if value.map(&:worktime).inject(:+) > 1
        break
      end
    end

As I could see the output in the console, when creating a new record, the hash 'ime_entries_by_date' in the validation method is empty, that's why the record is saved without problems even if the worktime exceeds the validation value of 1 day.
When updating the same record, the validation method catches well the error but does not refresh the hash values even after I modify the work time values to make it pass.
Any idea how to fix that?

Thank you