What can I do for model testing in Unit/Test on ruby

For model testing in Unit/test on ruby, I just test the DB saving of
all the tables?
Like this model
def self.get_latest_answers_for_person(person_id, limit=10)
    return nil if person_id.nil?
    return Answer.find_by_sql(['
      SELECT * FROM answers
      WHERE person_id = ?
      ORDER BY created_at DESC
      LIMIT ?;
      ', person_id, limit])
  end
I just test data in answer and users tables saving?
Help!