load_only as stated in
http://docs.sqlalchemy.org/en/latest/orm/loading_columns.html does the
following:
"An arbitrary set of columns can be selected as “load only” columns, which
will be loaded *while deferring all other columns* on a given entity..."
However, joined relationships are not affected by this, meaning that the
produced sql still joins all 'joined' relationships
Consider the following example:
import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Foo(Base):
__tablename__ = 'foo'
id = sa.Column(sa.Integer, primary_key=True)
bars = sa.orm.relationship("Bar", lazy="joined")
class Bar(Base):
__tablename__ = 'bar'
id = sa.Column(sa.Integer, primary_key=True)
foo_id = sa.Column(sa.Integer, sa.ForeignKey('foo.id'))
baz = sa.Column(sa.Integer)
e = sa.create_engine("sqlite://", echo=True)
session = sa.orm.Session(e)
session.query(Foo).options(sa.orm.load_only('id')).all()
The query produced is the following:
SELECT foo.id AS foo_id, bar_1.id AS bar_1_id, bar_1.foo_id AS bar_1_foo_id,
bar_1.baz AS bar_1_baz
FROM foo LEFT OUTER JOIN bar AS bar_1 ON foo.id = bar_1.foo_id
I understand that I can use the lazyload option to prevent the join but I
thought it would be nice if the load_only handled it out-of-the-box...
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