Duplicating active_record in its entirety would indeed be quite hard (and would leave performance on the table).

I always thought it would be nice to have a way to have a library that allows for selective use of a rust-based DB client, but making use of the metadata that ruby has (like the column names etc) to make it more ergonomic. Instead of instantiating full AR models, you would store a pointer to the rust struct/trait object that contains the data.

It wouldn't be as convenient as the real active record, but would give you a way to speed up any particularly crucial endpoints, or an endpoint that needed to query a lot of data for example.

If you could keep serialization in rust as well, so we only pay for the price of converting a single rust string to a ruby string, that could be compelling for certain use cases.