title: Django form: WARNING:root:Could not resolve form field
body: <p>I'm using django v1.8 in python 2.7</p>
  
  <p>In a table in <code>models.py</code> I have</p>
  
  <pre><code>class Diagnosis(models.Model):
      diagnosis_circumstances = models.CharField(max_length=150)
      diagnosis_circumstances_date = models.DateField('Date of diagnosis',null=True,blank=True)
      date_of_input= models.DateField(null=True,blank=True)
      pub_date = models.DateTimeField(auto_now=True)
      author = models.ForeignKey(User)
      patient = models.ForeignKey(Demographic)
  </code></pre>
  
  <p>When I'm trying to show <code>pub_date</code> field in <code>forms.py</code> like:</p>
  
  <pre><code>class DiagnosisForm(forms.ModelForm):
      def __init__(self, *args, **kwargs):
  
          super(DiagnosisForm, self).__init__(*args, **kwargs)
          self.helper=FormHelper(form=self)
          self.helper.field_class = 'col-md-8'
          self.helper.label_class = 'col-md-3'
  
          # self.helper.form_class = 'forms-horizontal'
          self.helper.layout = Layout(
              Fieldset(
                  '&lt;b&gt;Latest update&lt;/b&gt;',
                  Div(
                      Div('pub_date', css_class='col-md-8'),
                      css_class='row',
                  ),
              ),
              FormActions(
                  Submit('submit', "Save changes"),
                  Submit('cancel',"Cancel")
              ),
          )
          self.helper.form_tag = False
          self.helper.form_show_labels = True
  
      class Meta:
          model = Diagnosis
          exclude = ['patient', 'author']
          list_display = ('patient', 'pub_date', 'author')
  </code></pre>
  
  <p>I get the message: </p>
  
  <pre><code>WARNING:root:Could not resolve form field 'pub_date'.
  Traceback (most recent call last):
    File "C:\Python27\lib\site-packages\crispy_forms\utils.py", line 92, in render_f
  ield
      field_instance = form.fields[field]
  KeyError: u'pub_date'
  </code></pre>
  
  <p>Do you have any idea how to fix this?</p>