python向后分页 [英] python backward paging
问题描述
我试图根据google appengine文档的示例进行反向分页。
我的问题集中在这个例子上: p>
#设置。
q = Bar.query()
q_forward = q.order(Bar.key)
q_reverse = q.order(-Bar.key)
#网页前进。
bars,cursor,more = q_forward.fetch_page(10)
#获取相同的页面后退。
rev_cursor = cursor.reversed()
bars1,cursor1,more1 = q_reverse.fetch_page(10,start_cursor = rev_cursor)
基于这个例子,我创建了我自己的版本(像这样):
def get (self):
#testing =测试(t =a);
#testing.put();
#testing2 =测试(t =b);
#testing2.put();
#testing3 =测试(t =c);
#testing3.put();
#testing4 =测试(t =d);
#testing4.put();
#testing5 =测试(t =e);
#testing5.put();
cursor = ndb.Cursor.from_websafe_string(self.request.get(c))
如果光标:
q = Testing.query()
q_forward = q.order(Testing.key)
q_reverse = q.order(-Testing.key)
bars,next_cursor,more = q_forward.fetch_page(2,start_cursor = cursor )
rev_cursor = cursor.reversed()
bars1,prev_cursor,more1 = q_reverse.fetch_page(2,start_cursor = rev_cursor)
self.response.write(< a href = \?c =+ prev_cursor.to_websafe_string()+\>& laquo< / a>< br />)
else:
q =测试。 query()
q_forward = q.order(Testing.key)
q_reverse = q.order(-Testing.key)
bars,next_cursor,more = q_forward.fetch_page( 2)
self.response.write('Hello world!< br />')
for酒吧在酒吧:
self.response.write(bar.t +< br />)
self.response.write(< a href = \?c =+ next_cursor.to_websafe_string()+\>& raquo< / a>)
但我仍不明白为什么,它无法完美地回到上一页...
当我点击前页1到页面2时:
第1页:a,b
第2页:c,d p
但是当我点击后面时:
第2页:c,d
Page 1:b,c(应该是:a,b)
它使我困惑很多,因为有人在论坛上,可以使它基于这个例子工作,并没有人给他们的例子代码...
解决方案 问题是你使用 c
来引用一个向前和向前的游标。
$ $ $ $ $ $ c $ rev_cursor = cursor.reversed()
之前
bars1,prev_cursor,more1 = q_reverse。 fetch_page(2,start_cursor = rev_cursor)
等光标开始指向另一个方向,然后开始查询。
要查看更简单的测试,请定义相同的模型并预先填充一些数据:
from google.appengine.ext import ndb
class测试(ndb.Model):
t = ndb.StringProperty()
ndb.put_multi (t ='a'),Testing(t ='b'),Testing(t ='c')])
通过查询前两个元素来获得游标:
>>> q_forward = Testing.query().order(Testing.t)
>>> result,forward_cursor,_ = q_forward.fetch_page(2)
>>> print [value.t for value in result]
[u'a',u'b']
在反向查询中使用该游标而不反转它。
>>> ; reverse_cursor = forward_cursor.reversed()
>>> result,_,_ = q_reverse.fetch_page(2,start_cursor = reverse_cursor)
>>> print [value.t for value in result]
[u'b',u'a']
与反向查询之后进行反向查询:
i have tried to make a backward paging, based on the example of google appengine documentation
my question is focused on this example:
# Set up.
q = Bar.query()
q_forward = q.order(Bar.key)
q_reverse = q.order(-Bar.key)
# Fetch a page going forward.
bars, cursor, more = q_forward.fetch_page(10)
# Fetch the same page going backward.
rev_cursor = cursor.reversed()
bars1, cursor1, more1 = q_reverse.fetch_page(10, start_cursor=rev_cursor)
based on this example i create my own version (like this):
def get(self):
#testing = Testing(t="a");
#testing.put();
#testing2 = Testing(t="b");
#testing2.put();
#testing3 = Testing(t="c");
#testing3.put();
#testing4 = Testing(t="d");
#testing4.put();
#testing5 = Testing(t="e");
#testing5.put();
cursor = ndb.Cursor.from_websafe_string(self.request.get("c"))
if cursor:
q = Testing.query()
q_forward = q.order(Testing.key)
q_reverse = q.order(-Testing.key)
bars, next_cursor, more = q_forward.fetch_page(2, start_cursor=cursor)
rev_cursor = cursor.reversed()
bars1, prev_cursor, more1 = q_reverse.fetch_page(2, start_cursor=rev_cursor)
self.response.write("<a href=\"?c="+prev_cursor.to_websafe_string()+"\">«</a><br />")
else:
q = Testing.query()
q_forward = q.order(Testing.key)
q_reverse = q.order(-Testing.key)
bars, next_cursor, more = q_forward.fetch_page(2)
self.response.write('Hello world!<br />')
for bar in bars:
self.response.write(bar.t + "<br />")
self.response.write("<a href=\"?c="+next_cursor.to_websafe_string()+"\">»</a>")
but i still not understand why, it cannot back to previous page perfectly... when i click forward page 1 to page 2:
Page 1 : a, b Page 2 : c, d
but when i click backward:
Page 2: c, d Page 1: b, c (should be: a, b)
it confused me a lot, since somebody at forum, can make it work based on this example, and nobody give an example code from them...
The issue is that you are using c
to refer to a backwards and forwards cursor.
So, when you get a backwards cursor which has already been reversed, you are calling
rev_cursor = cursor.reversed()
before
bars1, prev_cursor, more1 = q_reverse.fetch_page(2, start_cursor=rev_cursor)
and so the cursor begins pointing in the other direction before beginning the query.
To see a simpler test of this, define the same model and pre-populate some data:
from google.appengine.ext import ndb
class Testing(ndb.Model):
t = ndb.StringProperty()
ndb.put_multi([Testing(t='a'), Testing(t='b'), Testing(t='c')])
get a cursor by querying for the first two elements:
>>> q_forward = Testing.query().order(Testing.t)
>>> result, forward_cursor, _ = q_forward.fetch_page(2)
>>> print [value.t for value in result]
[u'a', u'b']
use that cursor in a reverse query without reversing it
>>> reverse_cursor = forward_cursor.reversed()
>>> result, _, _ = q_reverse.fetch_page(2, start_cursor=reverse_cursor)
>>> print [value.t for value in result]
[u'b', u'a']
versus in a reverse query after doing so:
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