You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 20190917140739_create_check_to_keep_unique_absence_justifications_students.rb
    path of file: ./repos/i-diario/db/migrate
    
    the code of the file until where you have to start completion: class Cre
    
  def up
    execute <<-SQL
      ALTER TABLE absence_justifications_students
      ADD CONSTRAINT check_absence_justification_student_is_unique
      CHECK (
        check_absence_justification_student_is_unique(student_id, absence_justification_id, id, discarded_at)
      ) NOT VALID;
    SQL
  end