title: How to hide model field in Django Admin?
body: <p>I generate field automaticly, so I want to hide it from user. I've tried <strong>editable = False</strong> and hide it from <strong>exclude = ('field',)</strong>. All this things hide this field from me, but made it empty so I've got error: <em>null value in column "date" violates not-null constraint</em>. </p>
  
  <p>models.py:<br/>
  class Message(models.Model):</p>
  
  <pre><code>title = models.CharField(max_length = 100)
  text = models.TextField()
  date = models.DateTimeField(editable=False)
  user = models.ForeignKey(User, null = True, blank = True)
  main_category = models.ForeignKey(MainCategory)
  sub_category = models.ForeignKey(SubCategory)
  groups = models.ManyToManyField(Group)`
  </code></pre>
  
  <p>admin.py:</p>
  
  <pre><code>class MessageAdminForm(forms.ModelForm):
  
      def __init__(self, *arg, **kwargs):
          super(MessageAdminForm, self).__init__(*arg, **kwargs)   
          self.initial['date'] = datetime.now()
  
  class MessageAdmin(admin.ModelAdmin):
  
      form = MessageAdminForm
      list_display = ('title','user',)
      list_filter = ('date',)
  </code></pre>