CODEWITHSUNDEEP
pythongoogle-app-engineapp-engine-ndb
Harshal Patil
Harshal Patil

Reputation: 624

Query cursor for app engine in python using query iterator

I want to use pagination not by calling, 

greets, next_curs, more = Greeting.query().fetch_page(10, start_cursor=curs)

but by using query iterators. Because that way I can use tasklets to club together all consecutive 'gets' as described at, https://developers.google.com/appengine/docs/python/ndb/async#tasklets

Now in my code things looks like, 

class Book(ndb.Model):
    author = ndb.KeyProperty('a', kind=User, required=True)

author_key = ndb.Key(urlsafe=user_id)
books_query=Book.query(Book.author == author_key)
@ndb.tasklet
def callback(book):
     author = yield book.author.get_async()
     raise ndb.Return(book, author)

q_option = {'limit' : 2, 'produce_cursors' : True }
outputs = books_query.map(callback, **q_option)

Not sure where things are going wrong but if I try to do, 

books_query.iter().cursor_before()

or 

books_query.iter().cursor_after()

It fails with following trace,

raise datastore_errors.BadArgumentError('There is no cursor currently')
BadArgumentError: There is no cursor currently

Any elaborate example of using query iterators and tasklets with cursors would be really helpful to wrap my head around this.

Upvotes: 1

Views: 911

Answers (1)

Josh
Josh

Reputation: 603

books_query.iter().cursor_after() relies on .iter(**q_options), which creates a QueryIterator with its own parameters.

The problem is that there is no way to access the internal iterator used by .map().

See Guido's answer to a related question on the subject: https://stackoverflow.com/a/14150081/2380615

Upvotes: 1

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