title: 'Options' object has no attribute 'get_all_field_names'
body: <p>In my <code>django-admin</code>, I am trying to make a <code>model</code> non-editable. </p>
  
  <p>So, I am <code>overriding</code> a method <code>get_readonly_fields</code> of <code>admin.ModelAdmin</code>. </p>
  
  <p><code>Here is my Code</code></p>
  
  <pre><code>@admin.register(SMSTemplate)
  class SMSTemplateAdmin(admin.ModelAdmin):
      list_display=['title', 'json', 'note']
         formfield_overrides = {
         JSONField: {'widget': PrettyJSONWidget }
      }
  
      def has_delete_permission(self, request, obj=None):
          return False
  
      def get_readonly_fields(self, request, obj=None):
          return self.model._meta.get_all_field_names()
  </code></pre>
  
  <p>But I am facing an error. I am pasting the error here. </p>
  
  <pre><code>'Options' object has no attribute 'get_all_field_names'
  </code></pre>
  
  <p>Any Idea why ?</p>