Content Management Systems - MongoDB Applied Design Patterns - page 130

Since the full_slug field contains both hierarchical information (via the path) and chrono-
logical information, we can use a simple sort on the full_slug field to retrieve a threaded
view:

cursor = db . comments . find ({ 'node_id' : node_id })
cursor = cursor . sort ( 'full_slug' )
cursor = cursor . skip ( page_num * page_size )
cursor = cursor . limit ( page_size )

To support these queries efficiently, maintain two compound indexes on node_id, posted
and node_id, full_slug :

>>> >>> db . comments . ensure_index ([

...

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

>>> >>> db . comments . ensure_index ([

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

Operation: Retrieve comments via direct links

To directly retrieve a comment, without needing to page through all comments, we can select
by the slug field:

comment = db . comments . find_one ({
'node_id' : node_id ,
'slug' : comment_slug })

We can also retrieve a “subdiscussion,” or a comment and all of its descendants recursively,
by performing a regular expression prefix query on the full_slug field:

import import rre

subdiscussion = db . comments . find_one ({
'node_id' : node_id ,
'full_slug' : re . compile ( '^' + re . escape ( parent_full_slug )) })
subdiscussion = subdiscussion . sort ( 'full_slug' )

Since we've already created indexes on { node_id: 1, full_slug: 1 } to support retriev-
ing subdiscussions, we don't need to add any other indexes here to achieve good performance.