Q: is there an afterFieldSet event? about meteor-astronomy

[lukejagodzinski]

There was such event in Astronomy 1, but then every change had to be made using the doc.set("propName", value) method. In Astronomy 2, I've decided to no go this route. This feature was adding extra complexity and performance wise it wasn't great. There is also browser compatibility issue where setters and getters are not supported in all browsers.

In your situation, I would create some helper method that would not only set field's value but also recompute transient field. You could probably also class prototype by defining some setters (Object.defineProperty) but that's unlikely to work well in this scenario