Completely lazy loaded entity
Andrey Popp
is it possible to get completely lazy loaded entity? By completely lazy loaded
I mean no queries are done to database, so I have just instantiated object of
mapped class with only known attribute pk. If I access any other attribute
besides pk SQLAlchemy will load entire object state from database. Use case is
the following -- I have user's id encoded in cookie and want to have user
object attached to WSGI request, but if some of my views doesn't use it then it
shouldn't be queried from db.
Thanks!
Michael Bayer
it's not a built in use case, but to avoid any SQL whatsoever you'd need to make the object "persistent" in a fake way. Then you can get it in via merge():
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.orm.util import identity_key
from sqlalchemy.orm.attributes import instance_state
from sqlalchemy.ext.declarative import declarative_base
Base= declarative_base()
class X(Base):
__tablename__ = "x"
id = Column(Integer, primary_key=True)
data = Column(String)
e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
s = Session(e)
s.add_all([
X(data='x1'), X(data='x2'), X(data='x3')
])
s.commit()
s.close()
def make_a_fake_detached_x(id):
# x with clean dict
x = X()
# give it a key
instance_state(x).key = identity_key(X, (id,))
return x
# use merge() with load=False so no SQL is emitted
x2 = s.merge(make_a_fake_detached_x(2), load=False)
assert 'data' not in x2.__dict__
assert x2.data == 'x2'
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