Mongodb tail subdocuments - mongodb

Question

I have a collection with users. Each user has comments. I want to track for some specific users (according to theirs ids) if there is a new comment.
Tailable cursor I guess are what I need but my main problem is that I want to track subdocuments and not documents.
Sample of tracking documents in python:
db = Connection().my_db
coll = db.my_collection
cursor = coll.find(tailable=True)
while cursor.alive:
try:
doc = cursor.next()
print doc
except StopIteration:
time.sleep(1)
One solution is to run intervals every x time and see if the number of the comments has changed. However I do not find the interval solution very appealing. Is there any better way to track changes? Probably with tailable cursors.
PS: I have a comment_id field (which is an ObjectID) in each comment.
Small update:
Since I have the commect_id bson, I can store the biggest (=latest) one in each user. Then run intervals compare the bson if it's still the latest one. I don't mind not to be a precisely real time method. Even 10 minutes of delay is fine. However now I have 70k users and 180k comments but I worry for the scalability of this method.

Answer 1

This would be my solution. Evaluate if it fits your requirement -
I am assuming a data structure as follows
db.user.find().pretty()
{
"_id" : ObjectId("5335123d900f7849d5ea2530"),
"user_id" : 200,
"comments" : [
{
"comment_id" : 1,
"comment" : "hi",
"createDate" : ISODate("2012-01-01T00:00:00Z")
},
{
"comment_id" : 2,
"comment" : "bye",
"createDate" : ISODate("2013-01-01T00:00:00Z")
}
]
}
{
"_id" : ObjectId("5335123e900f7849d5ea2531"),
"user_id" : 201,
"comments" : [
{
"comment_id" : 3,
"comment" : "hi",
"createDate" : ISODate("2012-01-01T00:00:00Z")
},
{
"comment_id" : 4,
"comment" : "bye",
"createDate" : ISODate("2013-01-01T00:00:00Z")
}
]
}
I added createDate attribute to the document. Add an index as follows -
db.user.ensureIndex({"user_id":1,"comments.createDate":-1})
You can search for latest comments with the query -
db.user.find({"user_id":200,"comments.createDate":{$gt:ISODate('2012-12-31')}})
The time used for "greater than" comparison would be last checked time. Since you are using index, the search will be faster. You can follow the same idea of checking in for new comments in some interval.
You can also use UTC time stamp, instead of ISODate. That way you don't have to worry about bson data type.
Note that while creating index on createDate, I have specified descending index.
If you will have too many comments within a user document, over a period of time, I would suggest that, you move comments to a different collection. Use user_id as one of the attributes in the comment document. That will give a better performance in the long run.