Hi everybody,
In my db-setup, there is a one-to-many relationship with a backref that is
configured for eager loading (not the relation but the backref!). When I
access the child objects of the parent object, the query from sqlalchemy
joins back to the parent object (due to eager loading). In this case, the
join is unnecessary, since the parent object is already known.
Is there something I am missing or a way to avoid the join?
Cheers and thanks,
Martin
Simple test case:
from sqlalchemy import *
from sqlalchemy.orm import mapper, sessionmaker, relation, backref
engine = create_engine("postgres://testuser:testu...@localhost:5432/quicktest",
echo=True)
metadata = MetaData()
users_table = Table('users', metadata,
Column('id', Integer, primary_key=True),
Column('name', String(64)),
)
keywords_table = Table('keywords', metadata,
Column('id', Integer, primary_key=True),
Column('user_id', Integer, ForeignKey("users.id")),
Column('keyword', String(64))
)
class User(object):
def __init__(self, name):
self.name = name
class Keyword(object):
def __init__(self, keyword):
self.keyword = keyword
mapper(User, users_table, properties={
'keywords': relation(Keyword, backref=backref('user', lazy=False))})
mapper(Keyword, keywords_table)
metadata.drop_all(engine)
metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
user1 = User('kirk')
keyword1 = Keyword('enterprise')
keyword2 = Keyword('captain')
user1.keywords = [keyword1, keyword2]
session.add(user1)
session.commit()
queried_user1 = session.query(User).filter_by(name='kirk').one()
print queried_user1
print "--------------------------------------------------------------\n"
# This is where the unnecessary join happens
print queried_user1.keywords
print "--------------------------------------------------------------\n"
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