working:

    categories_string = fields.Function(fields.Char('Categories'), 'get_categories_string', searcher='search_categories_string')

    def get_categories_string(self, ids, name):
        if not ids:
            return []
        res = {}
        for party in self.browse(ids):
            res[party.id] = ', '.join(sorted(cat.rec_name for cat in party.categories))
        return res

development (and not working):

    def search_categories_string(self, name, clause):
        res = []
        value = clause[3]
        for cat, _ in self.categories:
            if isinstance(value, basestring) \
                    and cat.rec_name \
                    and value.upper() in cat.rec_name.upper():
                res.append(('categories', '=', cat.rec_name))
                value = value[len(cat.rec_name):]
                break
        res.append(('categories', clause[1], value))
        return res