How to Get Foreign Keys Horribly Wrong

This sort of thing hasn't really done much to make me like ORMs.

It seems like a lot of code to generate the tables in the first place and you STILL need to read the output scripts just to ensure the ORM isn't generating some garbage you didn't want.

That seems like a lot of extra effort when a simple migration service (such as liquibase) could do the same work running SQL directly. No question on "which indexes are getting created and why". No deep knowledge of Django interactions with sql. Instead, it's just directly running the SQL you want to run.

I’ve done a lot of interviewing and I’ve discovered that many devs (even experienced ones) don’t understand the difference between indexes and foreign keys.

My assumption is that people have used orms that automatically add the index for you when you create a relationship so they just conflate them all. Often they’ll say that a foreign key is needed to improve the performance and when you dig into it, their mental model is all wrong. The sense they have is that the other table gets some sort of relationship array structure to make lookups fast.

It’s an interesting phenomenon of the abstraction.

Don’t get me wrong, I love sqlalchemy and alembic but probably because I understand what’s happening underneath so I know the right way to hold it so things are efficient and migrations are safe.

I don't want to defend Django here, surely this should be categorized as a bug. But on the other hand, for this situation to come up you have to be the following:

- The kind of person to dive into the schema and worry about an unnecessary index

- Smart enough to heed Django's warnings and use `Meta.UniqueConstraint`

- Dumb enough to ignore Django's warnings and not use `Meta.Indexes`

I think it's funny that the kind of dev that 100% relies on the ORM and would benefit from this warning would probably never find themselves in this gritty optimization situation in the first place.

That being said, I enjoyed the article and learned something so maybe I'm the target audience and not them.

> Django will implicitly add an index on a ForeignKey field unless explicitly stated otherwise.

This is nice to know if you're using Django, but as important to note is that neither Postgres nor SQLAlchemy / Alembic will do this automatically.

How can we determine if an index can be satisfied by a constraint index?

For example, does the FK need to be the first field in a unique together?

Is this for real? I don’t know why anyone would deal with such amount of incidental complexity (django orm) when one can just use plain sql.