title: Django Model form with read only properties
body: <p>first of all let me explain the scenario: I'm working in a CRUD with Django 1.8.7, Python 2.7 and bootstrap as frontend. In the List of this CRUD when the user pressed the Add button a modal will open with a combo for the user to select what kind of item he wants to create. The user selects the option and the next form is loaded dynamically. So far it is working great. My problem is: the combo the user selected while in the list is one of the fields of this form and this field cannot be changed. It must be in the form just as text.</p>
  
  <p>Examples:</p>
  
  <p>Current form: </p>
  
  <p><a href="https://i.stack.imgur.com/6R5RQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6R5RQ.png" alt="Current form"></a></p>
  
  <p>What the form should be when the Url is : domain.com/adm/products/add?category_id=2</p>
  
  <p><a href="https://i.stack.imgur.com/5fUGh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5fUGh.png" alt="enter image description here"></a></p>
  
  <p>When the category_id is passed on the querystring I don't want to allow the user to edit this value.</p>
  
  <p>My question is: how can I achieve this? I'm using the CreateView for the view and ModelForm for the form.</p>
  
  <p>Thanks in advance for any tips</p>