cubicweb #5503548 rql2sql fails to translate a query [validation pending]

Some queries fails with a KeyError in rql2sql.

The following unittest reproduce the issue:

def test_rql2sql_ll(self):
    query = """
        Any X
        WHERE X is Event,
        EXISTS(X at_place A, A refpoint B, A is Place),
        X organized_by X_ORGANIZED_BY,
        X external_url X_EXTERNAL_URL?
    """

    with self.admin_access.repo_cnx() as cnx:
        john_eid = cnx.find('CWUser', login='john').one().eid

    args = {'D': john_eid}

    with self.repo.internal_cnx() as cnx:
        with cnx.security_enabled(False, False):
            cnx.execute(query, args)

The (hopefully) relevant part of the model:

class Event(WorkflowableEntityType):
    organized_by = SubjectRelation(
        ('CWUser', 'UnlishPage'), cardinality='1*', inlined=True,
        composite='object')

    external_url = SubjectRelation(
        'Link', __permissions__=__premium_relation_permissions__,
        cardinality='?*', composite='subject')

    at_place = SubjectRelation(
        'Place', __permissions__=__relation_permissions__,
        cardinality='?*', inlined=True)

class Place(EntityType):
    refpoint = GeoPoint()

The backtrace:

../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/session.py:371: in wrapper
    return func(cnx, *args, **kwargs)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/session.py:381: in check_open
    return func(cnx, *args, **kwargs)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/session.py:1019: in execute
    rset = self._execute(self, rql, kwargs, build_descr)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/devtools/__init__.py:814: in new_execute
    rset = base_execute(*args, **kwargs)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/querier.py:611: in execute
    results = plan.execute()
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/querier.py:192: in execute
    result = step.execute()
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/ssplanner.py:374: in execute
    result = source.syntax_tree_search(cnx, union, args, cachekey, inputmap)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/native.py:552: in syntax_tree_search
    sql, qargs, cbs = self._rql_sqlgen.generate(union, args, varmap)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:732: in generate
    sql = self.union_sql(union)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:753: in noparen_union_sql
    return '\nUNION ALL\n'.join(sqls)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:752: in <genexpr>
    for i, select in enumerate(union.children))
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:834: in select_sql
    needalias or needwrap)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:910: in _solutions_sql
    self._state.add_restriction(select.where.accept(self))
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/rql/utils.py:166: in accept
    return visit_method(self, *args, **kwargs)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:961: in visit_and
    part = c.accept(self)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/rql/utils.py:166: in accept
    return visit_method(self, *args, **kwargs)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:961: in visit_and
    part = c.accept(self)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/rql/utils.py:166: in accept
    return visit_method(self, *args, **kwargs)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:961: in visit_and
    part = c.accept(self)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/rql/utils.py:166: in accept
    return visit_method(self, *args, **kwargs)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:1057: in visit_relation
    sql = self._visit_outer_join_relation(relation, rschema)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:1201: in _visit_outer_join_relation
    outertype, condition)
../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:558: in replace_tables_by_outer_join
    self.mark_as_used_in_outer_join(leftalias, addpending=False)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <cubicweb.server.sources.rql2sql.StateInfo object at 0x7f8620d9c850>, tablealias = '_X'
addpending = False

    def mark_as_used_in_outer_join(self, tablealias, addpending=True):
        """Mark table of given alias as used in outer join. This must be called
            after `outer_tables[tablealias]` has been initialized.
            """
        # remove a table from actual_table because it's used in an outer join
        # chain
>       scope, tabledef = self.tables[tablealias]
E       KeyError: '_X'

../../../.virtualenvs/unlish-dev/local/lib/python2.7/site-packages/cubicweb/server/sources/rql2sql.py:522: KeyError