Readonly for existing items only in Django admin inline

Question

I have a tabular inline model in the Django admin. I need 1 of the fields to not be changeable after it has been created, but setting it as readonly (via readonly_fields) wh

Answer 1

I actually came across another solution that seems to work really well (I can't take credit for this, but link here).

You can define the get_readonly_fields method on your TabularInline and set the read only fields appropriately when there is an object (editing) vs when there is not one (creating).

def get_readonly_fields(self, request, obj=None):
    if obj is not None:  # You may have to check some other attrs as well
        # Editing an object
        return ('field_name', )
    else:
        # Creating a new object
        return ()

This has the effect of making your target field readonly when you're editing an exiting instance while allowing it to be editable when creating a new instance.

---

As pointed out below in the comment, this doesn't quite work as intended because the obj passed is is actually the parent... There's an old django ticket that discusses this here.