Self-referential relation fails silently if an entity references itself

[Former user]

(original reporter: ants) Self-referential relation fails silently if an entity references itself:

node = Node()
node.parent = node
session.add(node)
session.flush() # parent_id is inserted as None, no circular dependency error

[Former user]

(original author: ants) Example testcase

[Mike Bayer]

This is a dupe of #1449, and in response to that I added documentation at http://www.sqlalchemy.org/docs/05/mappers.html#rows-that-point-to-themselves-mutually-dependent-rows to explain what's going on here very explicitly.

I'm not sure offhand how this condition could be detected unless its a simple "if child is obj and post_update is False" in dependency.py. not sure if that covers it.

[Mike Bayer]

although the failure here is not exactly silent. it complained loudly.

[Mike Bayer]

this test case fails appropriately since the unit of work refactor in 0.6.