Array fields joining

Joining documents between two collections is an aggregation topic you explored extensively in Chapter 7, Joining Data Examples. However, sometimes, you only need to efficiently join two fields within the same document where at least one of the fields is an array. Let's look at how you can achieve this.

Scenario

You are developing a new dating website using a database to hold the profiles of all registered users. For each user profile, you will persist a set of the user's specified hobbies, each with a description of how the user says they conduct their pursuit. Each user's profile also captures what they prefer to do depending on their mood (e.g., happy, sad, chilling, etc.). When you show the user profiles on the website to a person searching for a date, you want to describe how each candidate user conducts their hobbies for each mood to help the person spot their ideal match.

Populating the sample data

First, drop any old versions of...