gh-ost is hardly useful for anyone outside of github.
It's predicated on the fact that you don't use foreign keys. Now why would someone use MySQL without FKs... is beyond me, but I'm sure they have their reasons.
I've read about their reasons, I just didn't want to open a discussion about it :D
Deciding to implement constraints on the application/business layer is something that github can probably do, in the same vein that Facebook can create their own PHP compiler.
I just think that you can't use these examples as effective arguments on whether someone should use FKs or not. I guess whatever brute-force solutions these companies would apply, on any given problem, would probably work one way or the other.
If you want to have referential integrity there's no other choice. Otherwise you create tables that have no relation to each other. No one stops you from doing this, but then you probably don't want an RDBMS in the first place.
Rails by default does not add a foreign key constraint at the database level when defining a relationship[1]. And once you have a fairly large rails app that uses this default it's somewhat tricky to migrate.
Django's ORM will create DB-level foreign keys on databases/table engines that support them. But it doesn't create many other types of database-level validation (mostly just UNIQUE and NOT NULL constraints).
Some of that is compatibility reasons; not every DB Django supports would allow creating all the desired types of constraints in the DB, so Django has no choice but to ship application-layer enforcement.
You can work around that with manually-created migrations (and I've worked some places that did this, in order to get particular constraint types that were needed at the DB level), but I don't know of a good generic solution to that problem.
Django 2.2 is going with a somewhat-manual approach to allow specifying richer types of constraints, so maybe that'll lead to progress.
The rationale is that non-trivial conditions on ActiveRecord model associations can be quite dynamic (possibly implying ruby code evaluation), therefore constraints are best handled at the model level, or else the constraints would only be partially checked at the db level, or even conflicting with the models (e.g if a constraint is to be enforced based on some condition evaluated at runtime, like a simple if clause, or when using STI or polymorphism).
I have no clue what you're talking about. That's an honest statement, not trying to be confrontational.
If you have constraints that can be enforced by the DB, you simply use the DB's constraints, because the DB guarantees they're gonna work 100% of the times and your data will be correct.
If they are more dynamic or require custom business logic... well you do it on the application layer. That's what everyone does.
I mean you can probably implement your own transactions, that doesn't mean that you should. And if you do, then just admit that there's no point in using an off-the-shelf RDBMS.
Is it not more accurate to say "by default rails generates migrations with the foreign key argument set to a non default value of true"?
By default the relationship constructors do not create foreign keys, if they did it would be redundant to include it in the generated output surely? At least that's what the docs seem to say.
Here are some of Github's reasons for not using them: https://github.com/github/gh-ost/issues/331
I think this is pretty normal for heavy MySQL users, honestly. When I was at Eventbrite we didn't use them for the same reasons.