[Sqlalchemy-tickets] Issue #3465: using version_id_col & version_id_generator: False makes SQLAlchemy emit bad SQL on relationship update (zzzeek/sqlalchemy)

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

New issue 3465: using version_id_col & version_id_generator: False makes SQLAlchemy emit bad SQL on relationship update
https://bitbucket.org/zzzeek/sqlalchemy/issue/3465/using-version_id_col-version_id_generator

tonymillion:

I have a model as such (in this case I'm using Flask-SQLAlchemy, but thats not relevant to the error):
```
class Person(BaseModel):
    <snip - stuff for model, id as primary key etc>
    purchase_uuid = db.Column(postgresql.UUID(as_uuid=True), default=uuid.uuid4)

    __mapper_args__ = {
        'version_id_col': purchase_uuid,
        'version_id_generator': False
    }
```
this works fine i.e. we can block a certain `UPDATES` unless purchase_uuid is being updated (taken from http://docs.sqlalchemy.org/en/latest/orm/versioning.html#programmatic-or-conditional-version-counters )

We also have another model

```
class Session(BaseModel):
    <snip - stuff for model, id as primary key etc>

    # the person who is represented by this Session
    person_id = db.Column(db.Integer, db.ForeignKey("people.id"), index=True)

    person = db.relationship("Person",
                             # backref=db.backref("sessions", lazy="dynamic"),
                             lazy="joined",
                             uselist=False,
                             foreign_keys=[person_id])
```

When I uncomment the backref and then create a session object and assign a person:

```
    session = Session()
    session.person = person
    db.session.add(session)
    db.session.commit()
```

I get the following:

```
ProgrammingError: (psycopg2.ProgrammingError) syntax error at or near "WHERE"
LINE 1: UPDATE people SET  WHERE people.id = 1 AND people.purchase_u...
                           ^
 [SQL: 'UPDATE people SET  WHERE people.id = %(people_id)s AND people.purchase_uuid = %(people_purchase_uuid)s RETURNING people.purchase_uuid'] [parameters: {'people_id': 1, 'people_purchase_uuid': UUID('e3a73745-0979-42e3-90c5-72e6a32219c7')}]
```

As you can see SQLAlchemy is outputting some invalid SQL i.e. `UPDATE people SET  WHERE`

If I comment out either the backref (the `sessions` relationship) or the `__mapper_args__` the problem goes away.