Make ActiveRecord#find return value typed

[gaborszakacs]

Motivation

Currently the return value of e.g. Post#find gets generated as untyped. We could catch some subtle bugs if it was typed.

Imagine a novice Rails dev who's not familiar with which finder method would return nil and which would raise exception if no record was found. Sorbet could warn e.g. if we tried to nil check on the return value of a find.

Implementation

I first thought that find always returns with a model, but it turned out it can be used to query multiple record at once so I went with a union type: either the model or an array of the models is returned.

(Some more background: rails-sorbet chose to support only the one model retrieval for find and introduced a new method for the multiple record use case.)

Tests

I've modified the existing test for the generated signature for #find.

[st0012]

@gaborszakacs Thank you for the PR!

[paracycle]

@gaborszakacs Thanks for the contribution, but I am afraid this is not the correct type for what find returns. find returns a Model instance if supplied a single argument, or (exclusive or), it returns an array of Model instances if supplied more than one argument. So the return type cannot be modelled by a union type, it is a discriminated union (or a method overload) which Sorbet does not support. So T.untyped is the best we can do, given those limitations.

Revert PR here: #1101

[gaborszakacs]

@paracycle Yes, you're right, sorry for pushing this in the wrong direction, I'm kind of new to the Sorbet type system.

One question, I guess we don't want to "force" the single argument use case as sorbet-rails does, but probably it's a good idea to mention in the documentation that find is not type-safe and suggest using find_by!(id: ...) instead. I suppose find is quite frequent in Rails codebases, probably worth pointing this out explicitly. Wdyt? I'm happy to open a PR, if you agree.