[Sqlalchemy-tickets] Issue #3839: "FlushError: Over 100 subsequent flushes" when deleting same object twice in 1.1 (zzzeek/sqlalchemy)

[Adrian <iss...@bi...>]

New issue 3839: "FlushError: Over 100 subsequent flushes" when deleting same object twice in 1.1
https://bitbucket.org/zzzeek/sqlalchemy/issues/3839/flusherror-over-100-subsequent-flushes

Adrian:

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

Base = declarative_base()


class Foo(Base):
    __tablename__ = 'foo'
    id = Column(Integer, primary_key=True)


e = create_engine('sqlite:///', echo=False)
Base.metadata.create_all(e)
s = Session(e)

s.add(Foo())
s.commit()

foo = s.query(Foo).first()
s.delete(foo)
s.flush()
s.delete(foo)
s.flush()
s.commit()
```

With SQLalchemy 1.0:
```
[adrian@blackhole:/tmp/test]> pip install -q 'sqlalchemy<1.1'
[adrian@blackhole:/tmp/test]> python satest.py
/tmp/test/lib/python2.7/site-packages/sqlalchemy/orm/persistence.py:925: SAWarning: DELETE statement on table 'foo' expected to delete 1 row(s); 0 were matched.  Please set confirm_deleted_rows=False within the mapper configuration to prevent this warning.
  (table.description, expected, rows_matched)
```

With SQLAlchemy 1.1:
```
[adrian@blackhole:/tmp/test]> pip install -Uq sqlalchemy
[adrian@blackhole:/tmp/test]> python satest.py
Traceback (most recent call last):
  File "satest.py", line 26, in <module>
    s.commit()
  File "/tmp/test/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 874, in commit
    self.transaction.commit()
  File "/tmp/test/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 461, in commit
    self._prepare_impl()
  File "/tmp/test/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 444, in _prepare_impl
    "Over 100 subsequent flushes have occurred within "
sqlalchemy.orm.exc.FlushError: Over 100 subsequent flushes have occurred within session.commit() - is an after_flush() hook creating new objects?
```

It looks like the second delete adds the object to `s.deleted` and it stays there forever. It also fails only during `s.commit()`. not during a normal `s.flush()`

While it can be considered a bug in my application that I end up deleting the same object twice, I don't think the 1.1 behavior is correct - if it should indeed be an error case instead of just a warning as in 1.0 the error should probably be somewhat clear.