satest.py

from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import *

Base = declarative_base()


class Foo(Base):
    __tablename__ = 'foo'
    id = Column(Integer, primary_key=True)


class Bar(Base):
    __tablename__ = 'bar'
    id = Column(Integer, primary_key=True)
    foo_id = Column(Integer, ForeignKey('foo.id'))
    foo = relationship(Foo, lazy=False, backref=backref('bars', lazy=False))


e = create_engine('sqlite:///', echo=True)
Base.metadata.create_all(e)
s = Session(e)

s.add(Foo())
s.commit()

print
print 'default'
print s.query(s.query(Foo).exists()).scalar()
print
print 'without eagerloads'
print s.query(s.query(Foo).enable_eagerloads(False).exists()).scalar()

xoutput.txt

2016-12-01 15:36:46,224 INFO sqlalchemy.engine.base.Engine SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
2016-12-01 15:36:46,224 INFO sqlalchemy.engine.base.Engine ()
2016-12-01 15:36:46,224 INFO sqlalchemy.engine.base.Engine SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
2016-12-01 15:36:46,225 INFO sqlalchemy.engine.base.Engine ()
2016-12-01 15:36:46,225 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("foo")
2016-12-01 15:36:46,225 INFO sqlalchemy.engine.base.Engine ()
2016-12-01 15:36:46,225 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("bar")
2016-12-01 15:36:46,225 INFO sqlalchemy.engine.base.Engine ()
2016-12-01 15:36:46,226 INFO sqlalchemy.engine.base.Engine
CREATE TABLE foo (
        id INTEGER NOT NULL,
        PRIMARY KEY (id)
)


2016-12-01 15:36:46,226 INFO sqlalchemy.engine.base.Engine ()
2016-12-01 15:36:46,226 INFO sqlalchemy.engine.base.Engine COMMIT
2016-12-01 15:36:46,227 INFO sqlalchemy.engine.base.Engine
CREATE TABLE bar (
        id INTEGER NOT NULL,
        foo_id INTEGER,
        PRIMARY KEY (id),
        FOREIGN KEY(foo_id) REFERENCES foo (id)
)


2016-12-01 15:36:46,227 INFO sqlalchemy.engine.base.Engine ()
2016-12-01 15:36:46,227 INFO sqlalchemy.engine.base.Engine COMMIT
2016-12-01 15:36:46,229 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2016-12-01 15:36:46,229 INFO sqlalchemy.engine.base.Engine INSERT INTO foo DEFAULT VALUES
2016-12-01 15:36:46,229 INFO sqlalchemy.engine.base.Engine ()
2016-12-01 15:36:46,229 INFO sqlalchemy.engine.base.Engine COMMIT

default
2016-12-01 15:36:46,231 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2016-12-01 15:36:46,231 INFO sqlalchemy.engine.base.Engine SELECT EXISTS (SELECT 1
FROM foo LEFT OUTER JOIN bar AS bar_1 ON foo.id = bar_1.foo_id) AS anon_1
2016-12-01 15:36:46,231 INFO sqlalchemy.engine.base.Engine ()
True

without eagerloads
2016-12-01 15:36:46,232 INFO sqlalchemy.engine.base.Engine SELECT EXISTS (SELECT 1
FROM foo) AS anon_1
2016-12-01 15:36:46,232 INFO sqlalchemy.engine.base.Engine ()
True