Validates presence of foreign key fails in nested form

I’m using accepts_nested_attributes as follows:

class Timesheet < ActiveRecord::Base
attr_accessible :status, :user_id, :start_date, :end_date,
has_many :activities, dependent: :destroy
has_many :time_entries, through: :activities
accepts_nested_attributes_for :activities, allow_destroy: true

class Activity < ActiveRecord::Base
attr_accessible :task_id, :timesheet_id, :time_entries_attributes
validates :task_id, presence: true, uniqueness: { scope:
belongs_to :timesheet
belongs_to :task
has_many :time_entries, order: :workdate, dependent: :destroy

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

As you see, I didn’t define any validation for timesheet_id presence in
Activity model. It works fine in the browser, but it is still possible
create an activity without timesheet_id assigned.

If I add:

validates :timesheet_id, presence: true

to the Activity model, I can’t any more create a timesheet via browser.
idea ?

Found the way to do that by using ‘inverse_of’ option (see below):