Content Management Systems - MongoDB Applied Design Patterns - page 135

{ '_id' : node [ '_id' ],
'num_comment_pages' : node [ 'num_comment_pages' ] },
{ '$inc' : { 'num_comment_pages' : 1 } })
db . comment_pages . update (
{ 'node_id' : node [ '_id' ],
'page' : node [ 'num_comment_pages' ] + 1 },
{ '$inc' : { 'count' : 1 },
'$push' : { 'comments' : comment } },
upsert = True )

There are a few things to note about this code:
The first update will only $push a comment if the page is not yet full.

If the last comment page is full, we need to increment the num_comment_pages
propertyinthenode(solongassome otherprocess hasnotalready incremented that
property).

W e also need to re-run the update to $push the comment onto the newly created
comment page. Here, we've dropped the count constraint to make sure the $push
goes through. (While it's technically possible that 100 other concurrent writers were
adding comments and the new page is already full, it's highly unlikely, and the ap-
plication works just fine if there happen to be 101 comments on a page.)

Tosupportthe update operations,weneedtomaintainacompoundindexon node_id,page
in the comment_pages collection:

>>> >>> db . comment_pages . ensure_index ([

... ... ( 'node_id' , 1 ), ( 'page' , 1 )])

Operation: View paginated comments

Thefollowingfunctiondefineshowtopaginatecommentswherethenumberofcommentson
a page is not known precisely (i.e., with roughly 100 comments, as in this case):

def def find_comments ( discussion_id , skip , limit ):

query = db . comment_pages . find (
{ 'node_id' : node_id } )
query = query . sort ( 'page' )
for for page iin query :