title: Django save method is overwriting field values
body: <p>I have a model that is overwriting a field value during the save opperation, I don't know if that happens during the communication with the database or its a pure Django problem.</p>
  
  <p>My bugged model:</p>
  
  <pre><code>class Classificacao(models.Model):
      item = models.ForeignKey(Item, verbose_name="Item", db_index=True, on_delete=models.CASCADE, db_column='seq_prod')
      is_valid = models.BooleanField(default=True)
      created_at = models.DateTimeField(auto_now_add=True)
      updated_at = models.DateTimeField(auto_now=True)
  </code></pre>
  
  <p>In order to identify the bug I overwrited the save method:</p>
  
  <pre><code>def save(self, *args, **kwargs):
      print(self.is_valid)
      super(Classificacao, self).save(*args, **kwargs)
      print(self.is_valid)
  </code></pre>
  
  <p>When I created an instance of the class by the following code the overwrited 'save' prints the above:</p>
  
  <pre><code>classificacao_nova = Classificacao.objects.create(
      item=item,
      created_at=datetime.datetime.now()
  )
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>True
  False
  </code></pre>
  
  <p>With debug mode on the following inser query was shown:</p>
  
  <pre><code>(0.001) QUERY = 'INSERT INTO "CALCULO_CLASSIFICACAO" ("SEQ_PROD", "IS_VALID", "CREATED_AT", "UPDATED_AT") VALUES (:arg4, :arg7, :arg0, :arg2) RETURNING "CALCULO_CLASSIFICACAO"."ID" INTO :arg1' - PARAMS = (1288767006, False, Oracle_datetime(2019, 9, 10, 10, 23, 2, 359822), Oracle_datetime(2019, 9, 10, 10, 23, 2, 359836), '&lt;django.db.backends.oracle.utils.InsertIdVar object at 0x7fb99f578be0&gt;'); args=(1288767006, False, Oracle_datetime(2019, 9, 10, 10, 23, 2, 359822), Oracle_datetime(2019, 9, 10, 10, 23, 2, 359836), &lt;django.db.backends.oracle.utils.InsertIdVar object at 0x7fb99f578be0&gt;)
  </code></pre>
  
  <p>Notice the <strong>False</strong> in the insert.</p>
  
  <p>Any ideas of what is going on?</p>
  
  <p>PS.: The database default for the is_valid column is 1.</p>