title: Django admin: how to predefine field values?
body: <p>I use Django 1.11.10 and python 3.6; I have <code>Category</code> model that has <code>name</code> and <code>parent</code>. Parent field links to itself. But when I create new <code>Category</code> model, I want choose parent from already created categories that have no parents. So how to predefine this list?</p>
  
  <pre><code>class Category(models.Model):
      name = models.CharField(max_length=50, unique=True)
      parent = models.ForeignKey('self', null=True, blank=True)
  
  ------
  
  class CategoryForm(forms.ModelForm):
      class Meta:
          model = Category
          fields = ['name', 'parent']
  
  class CategoryAdmin(admin.ModelAdmin):
      form = CategoryForm
  
  admin.site.register(Category, CategoryAdmin)
  </code></pre>