map

Applies transformation to this value once it resolves, so a row becomes the column the next step needs without a step of its own:

"edict_id" to edict.value().map { it.get<Int>("id") }

What the function throws arrives as a MappingException naming the step, the parameter and which map of the chain it was, with the original as its cause. An OctaviusException is rethrown as it was, with the same three on its path.