Add this code underneath the import statements on your ‘ArticleEdit’ Form:

categories = [
  (cat['name'], cat) for cat in app_tables.categories.search()
]

This creates a list of 2-tuples:

[
    ("First category name", <first_row_from_table>),
    ...,
    ("Final catgegory name", <final_row_from_table>)
]

The first element of each tuple is what will be displayed in the DropDown box. The second element of the tuple becomes the selected_value property of the DropDown if that option is selected.