MongoDB 3.2 introduced the
$lookup aggregation framework pipeline stage, which let you pull documents from a separate collection into your aggregation framework pipeline. Before MongoDB 3.6,
$lookup could only do left outer joins with equality matching. In other words, if you had a collection of users, a collection of stocks and a collection that mapped users to the stocks they hold,
$lookup could give you an array of stocks a user holds. But in MongoDB 3.2 and 3.4,
$lookup could not give you just the stocks that had gone up in price since the customer bought them.
Before MongoDB 3.6, tailing the MongoDB oplog was the only way to listen for changes to your MongoDB database. The oplog is a special collection that logs all inserts and updates that come in to your MongoDB server so other members of the replica set can copy them. Tools like Meteor and MoSQL were built on tailing the oplog. Unfortunately, the oplog was built primarily to support replication internally as opposed to being a user-facing feature. Change streams are a usability layer on top of the MongoDB oplog that make change detection in MongoDB much easier than tailing the oplog directly.
Strong support for arrays has always been one of MongoDB's killer features. For example, suppose you have a collection of blog posts where each document contains an array of comments as shown below. Before MongoDB 3.6, you could only update at most one element of the
comments array at a time because of limitations with the positional operator
$. Array filters in MongoDB 3.6 remove that limitation and add several more exciting features, like updating nested arrays.
One of the major reasons for the mongoose 5 release was the MongoDB driver removing support for the
authenticate() function. Up until Mongoose 4.11, that function was the only way mongoose supported doing authentication. In Mongoose 4.11.0, we added the annoying but necessary
useMongoClient deprecation warning, and in mongoose 5.0.0-rc0 we deleted 686 lines of legacy connection logic that removed the need for the
useMongoClient option. In addition, we made a couple related improvements to the connection API that will make mongoose much cleaner to work with: we made
mongoose.connect() always return a promise, and we added a global and connection-level
Mongoose 5.0.0-rc0 introduced several important changes to the way middleware works. The most pronounced difference is the ability to use promises and async/await with mongoose middleware. In addition, there are a couple more subtle changes that make the middleware API more consistent. In this article, I'll cover two changes. The first is that post hooks now always get flow control, even synchronous post hooks. The second is that query middleware is applied when the model is compiled for performance reasons.
This is the beginning of my first subscriber-only article on The Code Barbarian's Patreon. The article goes into much more depth than normal tech articles and has enough content to fill an ebook, so the whole article is only available to Patreon subscribers. If you like this article, please consider subscribing on Patreon to support future in-depth guides. The next article, "Building Distributed Locking with Node.js and MongoDB", ships on January 15, 2018!