[Sqlalchemy-tickets] Issue #4187: post_update fails if the parent object is deleted and the child isn't (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 4187: post_update fails if the parent object is deleted and the child isn't
https://bitbucket.org/zzzeek/sqlalchemy/issues/4187/post_update-fails-if-the-parent-object-is

Michael Bayer:

it sounds unbelievable but it's true.   a needless UPDATE is emitted that only recenly raises an error due to #3496, but it's always been there.   nothing else is needed:

```
#!python


from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    preferred_address_id = Column(ForeignKey("address.id"))

    preferred_address = relationship(
        "Address",
        primaryjoin=lambda: User.preferred_address_id == Address.id,
        post_update=True
    )


class Address(Base):
    __tablename__ = 'address'
    id = Column(Integer, primary_key=True)

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

sess = Session(e)

a1 = Address()
bart = User(preferred_address=a1)

sess.add(bart)

sess.flush()

sess.delete(bart)

sess.flush()


```

```
#!python


Traceback (most recent call last):
  File "test.py", line 38, in <module>
    sess.flush()
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 2243, in flush
    self._flush(objects)
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 2369, in _flush
    transaction.rollback(_capture_exception=True)
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/util/langhelpers.py", line 66, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/util/compat.py", line 187, in reraise
    raise value
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 2333, in _flush
    flush_context.execute()
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/unitofwork.py", line 391, in execute
    rec.execute(self)
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/unitofwork.py", line 542, in execute
    persistence.post_update(self.mapper, states, uow, cols)
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/persistence.py", line 234, in post_update
    mapper, table, update)
  File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/persistence.py", line 982, in _emit_post_update_statements
    (table.description, len(records), rows))
sqlalchemy.orm.exc.StaleDataError: UPDATE statement on table 'user' expected to update 1 row(s); 0 were matched.

```

no actual relationship cycle is needed, just the two objects.  the test_cycles test suite fails to test for this, here's a failing test:


```
#!diff

diff --git a/test/orm/test_cycles.py b/test/orm/test_cycles.py
index abd05067a..dc0be41bb 100644
--- a/test/orm/test_cycles.py
+++ b/test/orm/test_cycles.py
@@ -639,6 +639,28 @@ class OneToManyManyToOneTest(fixtures.MappedTest):
         sess.add(p)
         sess.flush()
 
+    def test_post_update_m2o_no_cascade(self):
+        person, ball, Ball, Person = (self.tables.person,
+                                      self.tables.ball,
+                                      self.classes.Ball,
+                                      self.classes.Person)
+
+        mapper(Ball, ball)
+        mapper(Person, person, properties=dict(
+            favorite=relationship(
+                Ball, primaryjoin=person.c.favorite_ball_id == ball.c.id,
+                post_update=True)))
+        b = Ball(data='some data')
+        p = Person(data='some data')
+        p.favorite = b
+        sess = create_session()
+        sess.add(b)
+        sess.add(p)
+        sess.flush()
+
+        sess.delete(p)
+        sess.flush()
+
     def test_post_update_m2o(self):
         """A cycle between two rows, with a post_update on the many-to-one"""
 

```

hoping this can be for 1.1.x however I am worried we might need #1063 for this to work, though "don't emit UPDATE if the object was deleted" seems simple...