get_history() call on relationship may interfere with other attributes' histories

[Roman Z.]

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.

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.

[Mike Bayer]

use the inspect api and use the .history attribute: http://docs.sqlalchemy.org/en/rel_1_0/orm/internals.html?highlight=instancestate#sqlalchemy.orm.state.AttributeState.history or if using get_history() send the correct PASSIVE flag through: http://docs.sqlalchemy.org/en/rel_1_0/orm/session_api.html#sqlalchemy.orm.attributes.get_history.params.passive

[Mike Bayer]

to clarify, your get_history() call is emiting a Query which is then autoflushing. Alternatively, block it within session.no_autoflush: http://docs.sqlalchemy.org/en/rel_1_0/orm/session_api.html?highlight=no_autoflush#sqlalchemy.orm.session.Session.no_autoflush

[Roman Z.]

Thanks for your quick answer.

What if I need the value of get_history(u, 'tags') (non-passive) too?

[Mike Bayer]

just turn off autoflush with the no_autoflush context manager. for critical sections where you need to inspect things with knowledge of the database, this is appropriate.