Question

The the post document looks like this:

{
  ...
  comments: [{
    _id:...
    body:...
    createDate:...
    },
    ...
  ]
}

How do I get recent 10 comments from the collection?

Answer 1

If your comments are always in a predictable order (i.e. newest first, or newest last), then you can use the $slice operator to return just a subset of the full comments field when querying:

test> db.foo.save({name: "hello", comments: [1, 2, 3, 4, 5]})
test> db.foo.find({}, {comments: {$slice: 3}})
{ "_id" : ObjectId("4ec7d1c8e72da9b6f31e2528"), "name" : "hello", "comments" : [ 1, 2, 3 ] }
test> db.foo.find({}, {comments: {$slice: -3}})
{ "_id" : ObjectId("4ec7d1c8e72da9b6f31e2528"), "name" : "hello", "comments" : [ 3, 4, 5 ] }

You can read more about controlling the returned fields at http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields