[lloeki]

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).

[rantanplan]

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.

[matthewmacleod]

Well, Rails is kind of an off-the-shelf solution to building web apps, so it makes some compromises to present a uniform process and API for some actions. One of those trade-offs is that the relationship between database tables in ActiveRecord is handled at the model/application layer instead of the database layer.

This has some benefits - it allows for a uniform definition of relationships regardless of database backend, allows for constraints that can’t be expressed by the database itself, and allows constraints to be used as a first-class concept for things like presenting error messages to users. But it also means that data integrity is not guaranteed - modifying records concurrently or outside of the application can result in a data model that’s valid according to the database schema but not according to the application model.

FWIW I exclusively use Postgres as my Rails database backend these days, and foreign key relationships are extremely easy to include in migrations. This still requires a companion definition in application code so that good error messages can be presented, but that seems acceptable to me. I’d hope that this eventually becomes the default for databases that support these keys.

[rantanplan]

Sure. I'm all for options and pluralism.

I am just that crazy person that believes that default options should err on the safety/integrity/consistency side.

I also love abstractions, but abstractions can't change their underlying fundamental reality. So I would like to be the one who makes the compromises and I'd like those compromises to be explicit rather than implicit.