cubicweb #3486149 have a proper API to declare META rtypes [open]

As of today, one can do as in the container cube/__init__.py

from cubicweb.schema import META_RTYPES
META_RTYPES.add('foo')

But this is error prone (it will add ghost rtypes for non-caring cubes).

prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>