[Sqlalchemy-tickets] Issue #3577: get_history() call on relationship may interfere with other attributes' histories (zzzeek/sqlalchemy)

[Roman Z. <iss...@bi...>]

New issue 3577: get_history() call on relationship may interfere with other attributes' histories
https://bitbucket.org/zzzeek/sqlalchemy/issues/3577/get_history-call-on-relationship-may

Roman Zimmermann:

I currently try to get the original values of certain attributes in `before_commit`. For that I call `get_history` on some attributes. It seems that the history of all attributes is flushed, when the history of the relationship is loaded.

Here is a full test-script.


```
#!python

from sqlalchemy import create_engine, Column, String, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import backref, relationship, sessionmaker
from sqlalchemy.orm.attributes import get_history, History
from sqlalchemy.sql.schema import ForeignKey

Base = declarative_base()


class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    name = Column(String)


class Tag(Base):
    __tablename__ = 'tag'
    id = Column(Integer, primary_key=True)
    user_id = Column(ForeignKey(User.id))
    tag = Column(String)
    user = relationship(User, backref=backref('tags'))


engine = create_engine('sqlite:///:memory:', echo=True)
Base.metadata.create_all(engine)
session = sessionmaker(bind=engine)()

u = User(name='old')
session.add(u)
session.commit()

getattr(u, 'name')
u.name = 'new'
assert get_history(u, 'name') == History(['new'], (), ['old'])
get_history(u, 'tags')
# Fails: History(added=(), unchanged=['new'], deleted=())
assert get_history(u, 'name') == History(['new'], (), ['old'])
```

It seems that the call `get_history(u, 'tags')` leads to the update being flushed.

Is there any way to consistently get the original value of an attribute from before the transaction has started?

This was tested using 1.0.8.