使用光标向后分页工作但缺少一个项目 [英] Backward pagination with cursor is working but missing an item

问题描述

从 寻找提供与 GAE 数据存储查询匹配的页面/项目计数/项目导航的想法/替代方法，我可以找到一个提示，说明如何通过 REVERSING ORDER 使用单个光标向后页面导航.

From looking for ideas/alternatives to providing a page/item count/navigation of items matching a GAE datastore query, I could find a hint how to backward page navigation with a single cursor by REVERSING ORDER.

class CursorTests(test_utils.NDBTest):

  def testFirst(self):
    class Bar(model.Model):
      value = model.IntegerProperty()

    self.entities = []
    for i in range(10):
        e = Bar(value=i)
        e.put()
        self.entities.append(e)

    q = Bar.query()
    bars, next_cursor, more = q.order(Bar.key).fetch_page(3)
    barz, another_cursor, more2 = q.order(-Bar.key).fetch_page(3, start_cursor=next_cursor)
    self.assertEqual(len(bars), len(barz))

不幸的是它因这个错误而失败.

Unfortunately it failed with this error.

回溯(最近一次调用最后一次):文件"/Users/reiot/Documents/Works/appengine-ndb-experiment/ndb/query_test.py",第 32 行，在 testFirst 中self.assertEqual(len(bars), len(baz)) AssertionError: 3 != 2

Traceback (most recent call last): File "/Users/reiot/Documents/Works/appengine-ndb-experiment/ndb/query_test.py", line 32, in testFirst self.assertEqual(len(bars), len(baz)) AssertionError: 3 != 2

是的，反向查询缺少边界中的项目.

Yes, an item in boundary is missing with reverse query.

bars = [Bar(key=Key('Bar', 1), value=0), Bar(key=Key('Bar', 2), value=1), Bar(key=Key('Bar', 3), value=2)] 
bars = [Bar(key=Key('Bar', 2), value=1), Bar(key=Key('Bar', 1), value=0)]

我该如何解决这个问题?

How can I fix this problem?

推荐答案

好的，这是官方的答案.需要反转"光标，如下:

Ok, here's the official answer. You need to "reverse" the cursor, as follows:

rev_cursor = cursor.reversed()

我自己也不知道.:-( 我会确保这显示在 fetch_page() 的文档中.

I did not know this myself. :-( I'll make sure this is shown in the docs for fetch_page().

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