[Answer]-Model inheritance + django admin

You created a form for News, that also exist in your DB since the model is not abstract, not for MyNews. Thus your current form has no field for the departments attribute, even if you add a widget with an input for it. Do the code bellow instead:

class MyNewsForm(NewsForm):
    class Meta:
        model = MyNews # instead of just News
        ...

What Django does in background is to create two relations: the cmsplugin_news_news stores all the News fields, and the news_news_departments stores your new field and is in one-to-one relation with the first relation.