What is the correct way to get the previous page of results given an NDB cursor?

Question

I\'m working on providing an API via GAE that will allow users to page forwards and backwards through a set of entities. I\'ve reviewed the section about cursors on the NDB

Answer 1

To make the example from the docs a little clearer let's forget about the datastore for a moment and work with a list instead:

# some_list = [4, 6, 1, 12, 15, 0, 3, 7, 10, 11, 8, 2, 9, 14, 5, 13]

# Set up.
q = Bar.query()

q_forward = q.order(Bar.key)
# This puts the elements of our list into the following order:
# ordered_list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

q_reverse = q.order(-Bar.key)
# Now we reversed the order for backwards paging: 
# reversed_list = [15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

# Fetch a page going forward.

bars, cursor, more = q_forward.fetch_page(10)
# This fetches the first 10 elements from ordered_list(!) 
# and yields the following:
# bars = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# cursor = [... 9, CURSOR-> 10 ...]
# more = True
# Please notice the right-facing cursor.

# Fetch the same page going backward.

rev_cursor = cursor.reversed()
# Now the cursor is facing to the left:
# rev_cursor = [... 9, <-CURSOR 10 ...]

bars1, cursor1, more1 = q_reverse.fetch_page(10, start_cursor=rev_cursor)
# This uses reversed_list(!), starts at rev_cursor and fetches 
# the first ten elements to it's left:
# bars1 = [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

So the example from the docs fetches the same page from two different directions in two differents orders. This is not what you want to achieve.

It seems you already found a solution that covers your use case pretty well but let me suggest another:

Simply reuse cursor1 to go back to page2.
If we're talking frontend and the current page is page3, this would mean assigning cursor3 to the 'next'-button and cursor1 to the 'previous'-button.

That way you have to reverse neither the query nor the cursor(s).