go-codec VS mapstructure

go-codec depends on reflect

Maybe https://github.com/mitchellh/mapstructure can do what you want? It has some options for Remainder Values and Omit Empty

I've tried using mapstructure to then marshal the map fields into a struct which mostly works (it struggles with times and custom time types which requires a workaround for each case), but this doesn't feel very idiomatic and requires two passes at marshaling.

Although some orders may have more records maybe a superset can be indentified that you can actually create a struct of it and after gathereing first all values into a map then convert it to a struct maybe using a library like https://github.com/mitchellh/mapstructure . this way you can at least isolate the non structured data only on the data extraction part and the rest of your application can work with well formed structs.

Alternatively, you could try https://github.com/mitchellh/mapstructure if you don't know what your incoming structure is

I'm doing it by validating a map[string]any first then putting it into a structure using mapstructure. It covers most use-cases and offers the most flexibility, at the expense of a bit of performance.

That said, it is possible to do this with JSON using something like https://github.com/tidwall/gjson or if you are fine with the switch statement but don't want to marshal and unmarshal again: https://github.com/mitchellh/mapstructure

https://github.com/mitchellh/mapstructure because we have JSON documents which contain rugged arrays ;-)

I've been using mapstructure.Decode to great effect, but currently can't figure out why a given mapping doesn't work. I'd appreciate it if someone could point out wtf I'm doing wrong or at least in the right direction:

What do you mean by "decode"? I've used https://github.com/mitchellh/mapstructure but that doesn't quite look like what you're doing.