title: check what field value changed in django model
body: <p>I parse data from a restful API call using django-rest-framework ModelSerializer. 
  Here is the code: </p>
  
  <pre><code>    url = "API URL HERE"
      r = requests.get(url)
      json = r.json()
      serializer = myModelSerializer(data=json, many=True)
      if serializer.is_valid():
          serializer.save()
  </code></pre>
  
  <p>Here is the modelSerializer:</p>
  
  <pre><code>class myModelSerializer(serializers.ModelSerializer):
      class Meta:
          model = MyModel
  </code></pre>
  
  <p>MYModel: </p>
  
  <pre><code>class MyModel(models.Model):
     City =  models.NullBooleanField(default=False, null=True)
     Status = models.CharField(max_length=100, null=True)
     stateName = models.CharField(max_length=50)
     marketingName = models.TextField(null=True)
     county = models.CharField(max_length=200, null=True)
  </code></pre>
  
  <p>My problem is I need to find out what field value changed from the last time I called the restful api and updated data. OR If there is any new records. How do I achieve this?</p>