The bug is passing string keys into business logic and not that the business logic expected atom keys, but you’re trying to change the latter instead of the first problem. User input (most often string keyed maps) should only ever land at a place where it’s Ecto.Changeset.cast or otherwise converted to an internal data structure. I would even say modifying those maps (e.g. add something to it) before “casting” to be a smell.

The other place for string keyed maps is if you need keys, which are user defined. But then your whole business logic needs to work with just string keys, because you never want to convert arbitrary user provided keys to atoms.

Exactly.

When I get input from the user, its usually that I validate it and typeconvert to elixir types as necessary. Then its either used for funcalls directly as bare value, or to create some other data. For what I have done so far, this other data is about 90% some struct that existed already before I decided to get some user input.

And in case of projects using ecto, well, at least 50% of them have a schema and a changeset function which makes validation/normalization very easy and straightforward.

Of course, sometimes other data structures make sense, then use it. And of course, if there are user provided relationships, just don’t use atom keys. But either way, deeper in your codebase nothing should access a map by a literal string, as either you know upfront what you expect or you don’t. And if you use a literal string, it feels as if you knew what you should get, but forgot to normalize. (And as always, there are exceptions to this “rule”, case by case)

I’ve only skimmed the implementation, but it seems as if it builds on a one-time creation of stringified/atomized counterkey with same value.

This seems to make things even worse…

One part of code updates atom key, another reads string key.

And even if you remember to re-key… Which should win if both exist?

The only solution are proper conversion to structs. No more arguing about string vs. atoms, just structs. Part of the contract, checked by dialyzer.