Returns subcategories (or ties if `target_object` is set)
        for the given parent category.

        :param str|None|list parent_aliases:
        :param ModelWithCategory|Model target_object:
        :param bool tied_only: Flag to get only categories with ties. Ties stats are stored in `ties_num` attrs.
        :return: a list of category objects or tie objects extended with information from their categories.

def get_categories(self, parent_aliases=None, target_object=None, tied_only=True):
        """Returns subcategories (or ties if `target_object` is set)
        for the given parent category.

        :param str|None|list parent_aliases:
        :param ModelWithCategory|Model target_object:
        :param bool tied_only: Flag to get only categories with ties. Ties stats are stored in `ties_num` attrs.
        :return: a list of category objects or tie objects extended with information from their categories.
        """

        single_mode = False
        if not isinstance(parent_aliases, list):
            single_mode = parent_aliases
            parent_aliases = [parent_aliases]

        all_children = []
        parents_to_children = OrderedDict()
        for parent_alias in parent_aliases:
            child_ids = self.get_child_ids(parent_alias)
            parents_to_children[parent_alias] = child_ids
            if tied_only:
                all_children.extend(child_ids)

        ties = {}
        if tied_only:
            source = OrderedDict()
            ties = self.get_ties_stats(all_children, target_object)
            for parent_alias, child_ids in parents_to_children.items():
                common = set(ties.keys()).intersection(child_ids)
                if common:
                    source[parent_alias] = common

        else:
            source = parents_to_children

        categories = OrderedDict()
        for parent_alias, child_ids in source.items():
            for cat_id in child_ids:
                cat = self.get_category_by_id(cat_id)
                if tied_only:
                    cat.ties_num = ties.get(cat_id, 0)

                if parent_alias not in categories:
                    categories[parent_alias] = []

                categories[parent_alias].append(cat)

        if single_mode != False:  # sic!
            return categories[single_mode]

        return categories