Accessing instance variables withing ActiveRecord model

Question

Been following Rails Testing Prescriptions. The Task table has a column named completed_at. Why can I only write to it with self.completed_at = date (completed_at = date fails) but I can access the attribute without self in the complete? method?

class Task < ApplicationRecord  
  belongs_to :project

  def mark_completed(date = Time.current)
    self.completed_at = date
  end

  def complete?
    completed_at.present?
  end
end

Thanks in advance