Rails - Jan 2007 - Render error message for NULL object

I''ve got an student_id column in my Attendance table. The ID is found
by
searching with the student''s IDN in the before_create method:

class Attendance < ActiveRecord::Base
  belongs_to :student

  validates_numericality_of :student_id, :only_integer => true

  def before_create
    self.student = Student.find_by_idn(student_id)
  end
end

This works just fine as long as the IDN that is entered is actually in
the Student table. If it is not, I get an error stating student_id is
NULL. How would I display "Student IDN not found" as an error message
in
my view? I''m new to programming and tried this but it does not work:

  def before_create
    s = Student.find_by_idn(student_id)
    if s = nil
      errors.add(:student_id, "is not a valid IDN")
    else
      self.student = s
    end
  end
end

Anyone have suggestions? Thanks for any help.

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