Expecting programmers to audit all of their code before a minor update is ridiculous.
It's not the responsibility of OSS maintainer to ensure your software still functions after an upgrade if you're not even willing to spend a modicum of effort to hold up your own end of that bargain. This isn't a big ask, it's why there are changelogs and release notes in the first place; they make it so you don't have to audit the code every time the version changes.
If you're not willing to do that and want unattended upgrades for all dependencies, then this is where the 'no warranty' aspect of that OSS license comes in.
The OSS maintainer has no responsibilities at all so you are right. But if anyone was to blame, it's certainly the library. It's outrageous to radically change a function while keeping the name the same. Because of exactly this issue. If they renamed it you would get an exception which is much nicer.
Some deps you can trust the owner and just carefully review the change log. Even that would have caught this issue, though I'm not sure I'd count this gem as trustworthy.
Maybe you are a very careful developer, but the vast majority out there is not.
Shipping an udate that will corrupt data if you don't read the changelog is very very dangerous.
I see why they did it. Having a method with the same name as in Active Record but with different behavior is also dangerous.
But they really could have handled this better.
These days, it's more a case of updating one thing at a time, and doing the research up front to see what I'm gaining from it or if I might as well stay on the current version. No point updating for the sake of it.
That's 5-10 minutes of up-front, preventative effort that might otherwise become hours of reactive firefighting, and as much time spent on damage control, if it got into production unchecked.
However, you can't rely on this. Many open-source projects fill a niche and become popular when it isn't the intention of the author. This causes a disconnect where the users expect the project to be for them, when the project is really for the author. Mongoid seems to be more in the former category (for the user).
Because some libraries are badly managed, you need to audit all of them.
TFA doesn't mention or acknowledge this aspect of the issue, but it did surprise me that they considered a version bump from 7.0.8 to 7.3.3 to be innocent looking.
In fact, these 'minor' releases actually appear to be quite significant and the issue in question in TFA was explicitly documented. [0][1]
I hate to say it but this was just a case of sloppy work on the startup's part. It took me less than 5 minutes to find that info.
[0] https://docs.mongodb.com/mongoid/current/tutorials/mongoid-u... [1] https://github.com/mongodb/mongoid/releases/tag/v7.1.0