title: Create Django model or update if exists
body: <p>I want to create a model object, like Person, if person's id doesn't not exist, or I will get that person object. </p>
  
  <p>The code to create a new person as following:</p>
  
  <pre><code>class Person(models.Model):
      identifier = models.CharField(max_length = 10)
      name = models.CharField(max_length = 20)
      objects = PersonManager()
  
  class PersonManager(models.Manager):
      def create_person(self, identifier):
          person = self.create(identifier = identifier)
          return person
  </code></pre>
  
  <p>But I don't know where to check and get the existing person object.</p>