MongoDB agregation: get next and previous element [duplicate] - mongodb

Question

I have a blog. On the individual post page I want to display a link to the previous, and if there is one, next post published in the bottom. The link should be the title of the specific post.
How do I do that the simplest way with Mongoose?
My current controller looks like this:
Post.findOne { slug : req.params.slug }, (err, post) ->
res.render "blog/show.jade", locals: title: "This post", post: post
And the schema looks like this:
PostSchema = new Schema(
title:
type: String
required: true
index: true
preamble: String
body: String
slug: String
createdAt: Date
updatedAt: Date
)

Answer 1

So let suppose you have schema like this:
{
_id,
text
}
I suppose that _id is mongo ObjectId, so we it contains post date and i can sort on it
Lets consider that i have opened current post with id equal to ObjectId( "43cc63093475061e3d95369d") (instead of this i will use curId) and i need to know next one and previous. Also lets consider that we need get all posts one by one ordered by created date descending:
Get next post you can like this:
db.posts.find({_id: {$gt: curId}}).sort({_id: 1 }).limit(1)
Get previous post you can like this:
db.posts.find({_id: {$lt: curId}}).sort({_id: -1 }).limit(1)
Few things:
If you don't use mongodb ObjectId above code will not work for you, but you can still use postDate instead of id and current post postDate instead of curId.
Take care about order when getting next/prev posts, to retrieve next post you need sort asc, to retrieve prev post you need sort desc.
I am not familiar with mongoose, so above scripts is mongodb shell scripts.

Answer 2

Find previous item:
Post.findOne({_id: {$lt: curId}}).sort({_id: -1}).exec(cb)
Find next item:
Post.findOne({_id: {$gt: curId}}).sort({_id: 1}).exec(cb)