[Sqlalchemy-tickets] Issue #3790: Setting unexpected type for relationship throws exception but modifies session state (zzzeek/sqlalchemy)

[Konsta V. <iss...@bi...>]

New issue 3790: Setting unexpected type for relationship throws exception but modifies session state
https://bitbucket.org/zzzeek/sqlalchemy/issues/3790/setting-unexpected-type-for-relationship

Konsta Vesterinen:

Consider the following model definition and data.
 
```
#!python

import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base, synonym_for

Base = declarative_base()


class Section(Base):
    __tablename__ = 'section'
    id = sa.Column(sa.Integer, primary_key=True)
    name = sa.Column(sa.String)


class Question(Base):
    __tablename__ = 'question'
    id = sa.Column(sa.Integer, primary_key=True)
    name = sa.Column(sa.String)
    section_id = sa.Column(sa.Integer, sa.ForeignKey(Section.id))
    section = sa.orm.relationship(Section, backref='questions')


engine = sa.create_engine('sqlite:///:memory:')
conn = engine.connect()

sa.orm.configure_mappers()
Base.metadata.create_all(conn)

Session = sa.orm.sessionmaker(bind=conn)
session = Session()

section = Section(questions=[Question(name='1'), Question(name='2')])
session.add(section)
session.commit()
```


Now if we did something like this accidentally
```
#!python

section.questions[0] = 'Some updated question name'
```
It throws Exception as expected but it also modifies session state so that calling

```
#!python

session.commit()
```

will result our given section having only one question. Maybe it would be better if set operations that threw an error would not affect the session state. 

I encountered this issue while doing stuff in console and later find out that I accidentally deleted one record.