title: make django model field read only or disable in admin while saving the object first time
body: <p>I want to disable the few fields from model in django admin while saving initially. </p>
  
  <pre><code>"&lt;input type="text" id="disabledTextInput" class="form-control" placeholder="Disabled input"&gt;"
  </code></pre>
  
  <p>like this.</p>
  
  <p>My model is:</p>
  
  <pre><code>class Blogmodel(models.Model):
      tag = models.ForeignKey(Tag)
      headline = models.CharField(max_length=255)
      image=models.ImageField(upload_to=get_photo_storage_path, null=True, blank=False)
      body_text = models.TextField()
      pub_date = models.DateField()
      authors = models.ForeignKey(Author)
      n_comments = models.IntegerField()
  </code></pre>
  
  <p>i want to disable the "headline" and "n_comments".
  i tried it in admin.py file, but its not disabling the fields on initial saving. But for editing the fields its working, it making the fields read only.</p>
  
  <p>in admin.py </p>
  
  <pre><code>class ItemAdmin(admin.ModelAdmin):
      exclude=("headline ",)
      def get_readonly_fields(self, request, obj=None):
          if obj:
              return ['headline']
          else:
              return []
  </code></pre>
  
  <p>Headling getting disabled but for edit only. i want to disable it at the time of object creation. i.e. first save.
  can anyone guide me for this?</p>