Linkbait title aside, it's actually a helpful example for directing a database novice on when to not use a document store. I could have used this post a few times in the past few years.
Maybe I'm too annoyed by the poorly chosen title. Or that I read that entire post and was thinking where's the punchline? On one hand, I credit the author for thinking things through. On the other, the fact that she unequivocally attributes this issue to MongoDB shows that she currently lacks the domain knowledge to consider appropriate use cases. It's not a MongoDB problem, it's a problem inherent to this data structure, and someone more well-versed in this topic would not conflate the issue...just as a decent IT person would not blame "Windoze" for the fact that she can't get good Wifi reception in the office.
OK, to be even more petty...I think what really aggravates me is how the OP says she's not a database expert -- which is a good disclosure, but self-evident -- but attempts to assert authority by saying "I build web applications...I build a lot of web applications"...Uh, OK, so what you're saying is that it's possible to be an experienced web developer and yet be a novice at data design?
If that was the angle of the OP, I'd give it five stars. Such sentiment cannot be overstated.
And maybe someone else, who has tackled enough difficult problems over time to evolve a nuanced and technically informed opinion of various data modeling and management options, should write the response I mentioned. I'd argue there are plenty examples of that material available already.
The OP, on the other hand, would be writing from the perspective of a professional user who might choose a tool off the shelf at the recommendation of a colleague, and whack it against the problem du jour to see if it works or not. This is a common enough approach that there is at least a chance that a followup would have some value. I can't really expect everyone who makes a living writing web applications to understand CS fundamentals, any more than I would expect it from chemical engineers or physicians. It is nice to be able to point representative members of that audience to an article that resonates with them, and not have to try to translate my opinions into similar language (with or without cat gifs).
Edit: I actually think Journeyman would be a more appropriate term than novice.
It is possible to be an experienced web developer without being an expert at databases, for some reasonable definition of 'expert', sure. I think so anyway. Do you find that aggravating?
Whether it's possible to be an experienced web developer while being a novice at either 'databases' or 'data design' (are those the same thing? you said the second, OP said the first) is open to debate I suppose, but is not implied by the OP.
The few things that stuck out at me:
* "Some folks say graph databases are more natural, but I’m not going to cover those here, since graph databases are too niche to be put into production." - So you did absolutely no research
* "What could possibly go wrong?" - the one line above the image saying those green boxes are the same gets lost. Give the image a caption, or better yet, use "Friends: User" to indicate type
* "Constructing an activity stream now requires us to 1) retrieve the stream document, and then 2) retrieve all the user documents to fill in names and avatars." - Yep, and since users are indexed by their ids, this is extremely easy.
* "What happens if that step 2 background job fails partway through?" - Write concerns. Or in addition to research, did you not read the mongo documents (write concern has been there at least since 2.2)
Finally, why not post the schemas they used? They make it seem like there are joins all over the place, when I mainly see, look at some document, retrieve users that match an array. Pretty simple mongo stuff, and extremely fast since user ids are indexed. Even though graph databases are better suited for this data, without seeing their schemas, I can't really tell why it didn't work for them.
I keep thinking "is it too hard to do sequential asynchronous operations in your code?".