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

According to this post this issue has been reported as a bug in Ticket15602.

A workaround would be to override the clean method of the inline model in forms.py and raise an error when an existing inline is changed:

class NoteForm(forms.ModelForm):
    def clean(self):
        if self.has_changed() and self.initial:
            raise ValidationError(
                'You cannot change this inline',
                code='Forbidden'
            )
        return super().clean()

    class Meta(object):
        model = Note
        fields='__all__'

The above gives a solution on the model level.

To raise an error when a specific field is changed, the clean_ method can help. For example, if the field is a ForeignKey called category:

class MyModelForm(forms.Form):
    pass  # Several lines of code here for the needs of the Model Form

# The following form will be called from the admin inline class only
class MyModelInlineForm(MyModelForm):
    def clean_category(self):
        category = self.cleaned_data.get('category', None)
        initial_value = getattr(
            self.fields.get('category', None), 
            'initial', 
            None
        )
        if all(
            (
                self.has_changed(),
                category.id != initial_value,
            )
        ):
            raise forms.ValidationError(
                _('You cannot change this'),
                code='Forbidden'
            )
        return category


    class Meta:
        # Copy here the Meta class of the parent model