spatial database - QGIS Value Map Edit Widget with properties dependent on another field

I'm having a problems with Edit Widgets in QGIS. I'm working on a GIS Database for archaeological data, where every site is represented by a polygon. In the attributes there is a field for the era which that site belongs to. I made this with Value Map, so as to make data processing easier and redundancy-free(er). My problem is that I'm trying to add a "sub-era" field as well to show if a site belongs to a certain period in a given era.

Naturally I want to use the same type of edit widget, but instead of listing every period for the whole database I thought I would make the period-field's value range dependent on the era selected in the previous field. For example: I have a site that belongs to the Roman Age, and I choose this at the era, so then in the period field only the periods for the Roman Age are shown (1st century, 2nd century, etc.), and the periods relevant to other eras are not.

Is this possible somehow in QGIS?

Thanks for the answers in advance!