[web/facet] Use an Exists node in HasRelationFacet
The generated RQL now includes an EXISTS clause. For instance, one gets:
::
DISTINCT Any WHERE X is CWUser, EXISTS(X in_group A)
instead of:
::
DISTINCT Any WHERE X is CWUser, X in_group A
for a HasRelationFacet with ``rtype = 'in_group'``, which is more appropriate
for testing the existence of the relation.
Add a test for this facet along the way.
from cubicweb.schemas.base import in_group, CWSourceSchemaConfig
# copy __permissions__ to avoid modifying a shared dictionary
in_group.__permissions__ = in_group.__permissions__.copy()
in_group.__permissions__['read'] = ('managers',)
cw_for_source = CWSourceSchemaConfig.get_relation('cw_for_source')
cw_for_source.__permissions__ = {'read': ('managers', 'users'),
'add': ('managers',),
'delete': ('managers',)}