MongoDB get last 10 activities - mongodb

Question

In My social network I want to get the feed for member A , member A is following lets say 20 category/member.
when a category/member(followed by member A) do an activity it is inserted into a collection called recent_activity :
{
"content_id": "6", // content id member A is following
"content_type_id": "6",// content type (category , other member)
"social_network_id": "2", // the action category did (add/like/follow)
"member_id": "51758", //Member A
"date_added": ISODate("2014-03-23T11:37:03.0Z"),
"platform_id": NumberInt(2),
"_id": ObjectId("532ec75f6b1f76fa2d8b457b"),
"_type": {
"0": "Altibbi_Mongo_RecentActivity"
}
}
I want when member A login into the system to get last 10 activities for the categories/member
my problem :
How to get Only 10 activities for all categories/members.
It is better to do it in one query or to do a for loop.

Answer 1

For this use case, I'd suggest to invert the logic and keep a separate object of the last 10 activities for member A that is kept up-to-date all the time. While that solution is more write-heavy, it makes reading trivially simple and it can be extended very easily. I'd like to blatantly advertise a blog post I wrote a while ago about news feeds with mongodb which outlines this approach.
This 'fan-out' approach might seem overly complex at first, but when you think about importance filtering / ranking (a la facebook), push messages for particularly important events (facebook, twitter) or regular digest emails (practically all), you will get one location in your code to perform all this logic.

Answer 2

I think I commented that T'm not really seeing the selection criteria. So if you are "outside" of a single collection, then you have problems. But if your indicated fields are the things you want to "filter" by, then just do this:
db.collection.find({
"social_network_id": "2",
"content_type_id": "6",
"content_id": "6",
"member_id": { "$ne": "51758" }
})
.sort({ "$natural": -1 })
.limit(10);
So what does that do? You match the various conditions in the data to do the "category match" (if I understood what was meant), then you make sure you are not matching entries by the same member.
The last parts do the "natural" sort. This is important because the ObjectId is monotinic, or math speak for "ever increasing". This means the "newest" entries are always the "highest" value. So descending order is "latest" to "oldest".
And the very final part is a basic "limit". So just return the last 10 entries.
As long as you can "filter" within the same collection in whatever way you want, then this should be fine.