Sharing this because I wish plone.restapi did this out of the box, and I find it useful for my own purposes — you can customize @navigation service to behave like @search with regard to the additional metadata_fields function by making override of Navigation adapter and layer-specific override of the NavigationGet classes as follows:
from plone.restapi.serializer.summary import DefaultJSONSummarySerializer
from plone.restapi.services.navigation import get as _get
class Navigation(_get.Navigation):
"""
Custom navigation adapter, customizes item metadata via
customize_entry hook-point.
"""
def customize_entry(self, item, brain):
if 'metadata_fields' in self.request.form:
# get summary serialization from plone.restapi summary serializer,
# which is designed for search, but really should in ideal world
# also work for navigation. Here we merge summary data when
# 'metadata_fields' param is in the request querystring:
# -- adapt catalog brain to summary metadata component: --
summary = DefaultJSONSummarySerializer(brain, self.request)
item.update(summary()) # merge summary k/v pairs into item dict
class NavigationGet(_get.NavigationGet):
"""
Custom service to use custom Navigation implementation
subclasses base from plone.restapi, implements customize_entry()
method for purposes of metadata injection into resulting items
in navigation JSON.
"""
def reply(self):
navigation = Navigation(self.context, self.request)
return navigation(expand=True)["navigation"]