SET NULL is not set for many-to-many self-relationship tables

[ichorid]

While trying to delete an entry with bulk=True, there is a FOREIGN KEY constraint failed message.
Apparently, Pony does not set the SET NULL action in the intermediate table.
That is peculiar because it sets this for regular (not intermediate) tables, as can be seen with
PRAGMA foreign_key_list(db_name);

Minimal example:

from pony import orm
from pony.orm import db_session


db = orm.Database()

class TestClass1(db.Entity):
    rowid = orm.PrimaryKey(int, auto=True)
    friend = orm.Set("TestClass1", reverse="friend")

db.bind(provider='sqlite', filename=":memory:", create_db=True)
db.generate_mapping(create_tables=True)

if __name__ == "__main__":
    with db_session:
        c1 = TestClass1()
        c2 = TestClass1(friend=c1)

    with db_session:
        print TestClass1.select()[:]
        TestClass1.select().delete(bulk=True)

Results in

Traceback (most recent call last):
  File "./pony_bug_self.py", line 21, in <module>
    TestClass1.select().delete(bulk=True)
  File "/home/user/.local/lib/python2.7/site-packages/pony/orm/core.py", line 5922, in delete
    cursor = database._exec_sql(sql, arguments)
  File "/home/user/.local/lib/python2.7/site-packages/pony/orm/core.py", line 941, in _exec_sql
    connection = cache.reconnect(e)
  File "/home/user/.local/lib/python2.7/site-packages/pony/orm/core.py", line 1771, in reconnect
    if not provider.should_reconnect(exc): reraise(*sys.exc_info())
  File "/home/user/.local/lib/python2.7/site-packages/pony/orm/core.py", line 939, in _exec_sql
    try: new_id = provider.execute(cursor, sql, arguments, returning_id)
  File "<auto generated wrapper of execute() function>", line 2, in execute
  File "/home/user/.local/lib/python2.7/site-packages/pony/orm/dbapiprovider.py", line 58, in wrap_dbapi_exceptions
    except dbapi_module.IntegrityError as e: raise IntegrityError(e)
pony.orm.dbapiprovider.IntegrityError: FOREIGN KEY constraint failed

[ichorid]

🎉