validates the presence of association when using nested forms

Why none of the validations work in update mode when using the below model relation and nested forms:


class Timesheet < ActiveRecord::Base

has_many :activities, dependent: :destroy

has_many :time_entries, through: :activities

validates_associated :activities, length: { minimum: 1 }

validates_presence_of :activities



class Activity < ActiveRecord::Base

class Activity < ActiveRecord::Base

attr_accessible :task_id, :timesheet_id, :time_entries_attributes

validates :task_id, presence: true

belongs_to :timesheet

belongs_to :task

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


If when updating a timesheet I remove an activity, the timesheet will be updated with no problems instead of raising validation error on activity.

Any idea? Thank you.