def gen_tree(self, category_strs):
    ''' takes an array of category strings eg:
            ["Heim & Garten > Rasen & Garten > Bewässerungssysteme",
            "Heim & Garten > Rasen & Garten > Gartenbau"]
    and returns an category tree dict eg:
            {
                "root": {
                    "children:": {
                        "Heim & Garten": {
                            "slug": {
                                "heim-garten"
                            }
                            "children": {
                                "Rasen & Garten": {
                                    "slug": {
                                        "rasen-garten"
                                    }
                                    "children": {
                                        "Bewässerungssysteme": {},
                                        "Gartenbau": {}
                                    }
                                }
                            }
                        }
                    }
                }
            }

    '''
    def merge(subcat, current_obj):
        # if new
        if subcat not in current_obj['children']:
            # go a level deeper
            children = {'slug': slugify(subcat), 'children': {}}
            # append
            current_obj['children'][subcat] = children
        else:
            # go a level deeper
            children = current_obj['children'][subcat]
        return children

    # init
    tree = {'root': {'children': {}}}

    for category_str in category_strs:
        # move 'pointer' to the beginning
        current_obj = tree['root']
        splitted = category_str.split(' > ')
        for category in splitted:
            current_obj = merge(category, current_obj)

    return tree