Long time Rails dev here. I believe not. The Rails core devs have long known that monkey patching is not thread safe, and blows Ruby method caches, so they already reserve monkey patching for initialization time.

I feel that the definition of "monkey patching" doesn't (or shouldn't) cover what Rails does in AR. Monkey patch suggests a change of behaviour over what was initially intended, however dynamic method generation in AR is very-much part of the intended API.

That said, there are plenty of Rails apps that do monkey patch Rails internals, for better or worse.

Do you have something specific in mind? From my understanding of ActiveRecord, most of the method generation should happen the first time the class is defined.

Closer to the first time it's used. Schema gets queried and methods get added the moment you do the first query on the model normally.

This is not true in production, where eager loading is always on.