How to use cursors for search in gae? - google-app-engine

Question

When I RTFM, I can't understand how to specify paginated searches using the technique described in the manual. Here's my code:
def find_documents(query_string, limit, cursor):
try:
subject_desc = search.SortExpression(
expression='date',
direction=search.SortExpression.DESCENDING,
default_value=datetime.now().date())
# Sort up to 1000 matching results by subject in descending order
sort = search.SortOptions(expressions=[subject_desc], limit=1000)
# Set query options
options = search.QueryOptions(
limit=limit, # the number of results to return
cursor=cursor,
sort_options=sort,
#returned_fields=['author', 'subject', 'summary'],
#snippeted_fields=['content']
)
query = search.Query(query_string=query_string, options=options)
index = search.Index(name=_INDEX_NAME)
# Execute the query
return index.search(query)
except search.Error:
logging.exception('Search failed')
return None
class MainAdvIndexedPage(SearchBaseHandler):
"""Handles search requests for comments."""
def get(self):
"""Handles a get request with a query."""
regionname = 'Delhi'
region = Region.all().filter('name = ', regionname).get()
uri = urlparse(self.request.uri)
query = ''
if uri.query:
query = parse_qs(uri.query)
query = query['query'][0]
results = find_documents(query, 50, search.Cursor())
next_cursor = results.cursor
template_values = {
'results': results,'next_cursor':next_cursor,
'number_returned': len(results.results),
'url': url, 'user' : users.get_current_user(),
'url_linktext': url_linktext, 'region' : region, 'city' : '', 'request' : self.request, 'form' : SearchForm(), 'query' : query
}
self.render_template('indexed.html', template_values)
The code above works and does a search but it doesn't page the result. I wonder about the following code in the manual:
next_cursor = results.cursor
next_cursor_urlsafe = next_cursor.web_safe_string
# save next_cursor_urlsafe
...
# restore next_cursor_urlsafe
results = find_documents(query_string, 20,
search.Cursor(web_safe_string=next_cursor_urlsafe))
What is next_cursor used for? How do I save and what is the purpose of saving? How do I get a cursor in the first place? Should the code look something like this instead, using memcache to save an restore the cursor?
class MainAdvIndexedPage(SearchBaseHandler):
"""Handles search requests for comments."""
def get(self):
"""Handles a get request with a query."""
regionname = 'Delhi'
region = Region.all().filter('name = ', regionname).get()
uri = urlparse(self.request.uri)
query = ''
if uri.query:
query = parse_qs(uri.query)
query = query['query'][0]
# restore next_cursor_urlsafe
next_cursor_urlsafe = memcache.get('results_cursor')
if last_cursor:
results = find_documents(query_string, 50,
search.Cursor(web_safe_string=next_cursor_urlsafe))
results = find_documents(query, 50, search.Cursor())
next_cursor = results.cursor
next_cursor_urlsafe = next_cursor.web_safe_string
# save next_cursor_urlsafe
memcache.set('results_cursor', results.cursor)
template_values = {
'results': results,'next_cursor':next_cursor,
'number_returned': len(results.results),
'url': url, 'user' : users.get_current_user(),
'url_linktext': url_linktext, 'region' : region, 'city' : '', 'request' : self.request, 'form' : SearchForm(), 'query' : query
}
self.render_template('indexed.html', template_values)
Update
From what I see from the answer, I'm supposed to use an HTTP GET query string to save the cursor but I still don't know exactly how. Please tell me how.
Update 2
This is my new effort.
def get(self):
"""Handles a get request with a query."""
regionname = 'Delhi'
region = Region.all().filter('name = ', regionname).get()
cursor = self.request.get("cursor")
uri = urlparse(self.request.uri)
query = ''
if uri.query:
query = parse_qs(uri.query)
query = query['query'][0]
logging.info('search cursor: %s', search.Cursor())
if cursor:
results = find_documents(query, 50, cursor)
else:
results = find_documents(query, 50, search.Cursor())
next_cursor = None
if results and results.cursor:
next_cursor = results.cursor.web_safe_string
logging.info('next cursor: %s', str(next_cursor))
template_values = {
'results': results,'cursor':next_cursor,
'number_returned': len(results.results),
'user' : users.get_current_user(),
'region' : region, 'city' : '', 'request' : self.request, 'form' : SearchForm(), 'query' : query
}
I think that I've understood how it's supposed to work with the above, and it's outputting a cursor at the first hit so I can know how to get the cursor in the first place. This is clearly documented enough. But I get this error message: cursor must be a Cursor, got unicode

Answer 1

No, you should not use memcache for that, especially with a constant key like 'results_cursor' - that would mean that all users would get the same cursor, which would be bad.
You are already passing the cursor to the template context (although you should be converting to the web_safe_string as you do in the second example). In the template, you should ensure that the cursor string is included in the GET parameters of your "next" button: then, back in the view, you should extract it from there and pass it into the find_documents call.
Apart from the memcache issue, you're almost there with the second example, but you should obviously ensure that the second call to find_documents is inside an else block so it doesn't overwrite the cursor version.