Cursor.py

class Cursor(object):
    """
    只是翻页的结构, 原始值有三种: 0, None, 其他
    若是 0, 其实是上次返回时指示已没有结果的意思, 应返回空
    若是 None, 表示是第一次请求, 应使用默认参数对请求做出响应
    若是其他值, 正数表示像后翻, 负数表示向前翻
    """
    DIRECTION_NEXT = 'next'
    DIRECTION_PREVIOUS = 'previous'
    DIRECTION_NONE = None

    def __init__(self, direction, position):
        self.direction = direction
        self.position = position

    @classmethod
    def parse(cls, request_handler):
        cursor_value = request_handler.get_argument_int('cursor', default=None)

        if cursor_value is None:
            cursor_direction = cls.DIRECTION_NEXT
            cursor_position = None

        elif cursor_value == 0:
            cursor_direction = None
            cursor_position = None

        else:
            cursor_direction = cls.DIRECTION_NEXT if cursor_value > 0 else cls.DIRECTION_PREVIOUS
            cursor_position = abs(cursor_value)

        cursor = Cursor(cursor_direction, cursor_position)
        return cursor

    @classmethod
    def get_records(cls, session, cursor, page_size, filters, record_obj):
        """
        按照 cursor 获得一页结果. 返回 records, cursor_previous, cursor_next

        @param session: sqlalchemy session
        @param cursor: Cursor obj
        @param page_size: 页大小
        @param filters: 一个 list, 条件过滤器
        @param record_obj: sqlalchemy 对象
        """
        cursor_previous = 0
        cursor_next = 0

        the_records = []

        if cursor.direction == cls.DIRECTION_NEXT:
            # 下一页
            if cursor.position:
                filters.append(record_obj.id < cursor.position)

            the_records = session.query(record_obj).filter(
                *filters
            ).order_by(record_obj.id.desc())[:page_size]

            if cursor.position and the_records:
                # 不是请求第一页, 前页可能有结果
                cursor_previous = -the_records[0].id

            if len(the_records) == page_size:
                # 结果满了一页了, 后页可能有结果
                cursor_next = the_records[-1].id

        elif cursor.direction == cls.DIRECTION_PREVIOUS:
            # 上一页
            filters.append(record_obj.id > cursor.position)

            top_n_records = session.query(record_obj).filter(
                *filters
            ).order_by(record_obj.id.asc()).limit(page_size).subquery()

            the_records = session.query(record_obj).select_entity_from(
                top_n_records
            ).order_by(record_obj.id.desc()).all()

            if len(the_records) == page_size:
                # 结果满了一页了, 前页可能有结果
                cursor_previous = -the_records[0].id

            if the_records:
                # 后面的结果不能确定有没有...
                cursor_next = the_records[-1].id

        return the_records, cursor_previous, cursor_next