Question

我想在两个共享相同模型的数据库之间迁移一些数据。这是我的代码：

from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

Base = declarative_base()


class Person(Base):
    __tablename__ = 'person'
    id = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)


# Create engines
source_engine = create_engine('sqlite:///source.db')
mirror_engine = create_engine('sqlite:///mirror.db')

# Create tables
Base.metadata.create_all(bind=source_engine)
Base.metadata.create_all(bind=mirror_engine)

# Create sessions
SourceSession = sessionmaker(bind=source_engine)
source_session = SourceSession()
MirrorSession = sessionmaker(bind=mirror_engine)
mirror_session = MirrorSession()

# Add data
source_session.add(Person(name="James"))
source_session.commit()

# Migrate data
results = source_session.query(Person).all()
for row in results:
    mirror_session.add(row)  # This line produces an error
mirror_session.commit()

# Close sessions
source_session.close()
mirror_session.close()

迁移行mirror_session.add(row)会产生以下错误：

sqlalchemy.exc.InvalidRequestError: Object '<Person at 0x280a6b29898>' is already attached to session '1' (this is '2')

Answer 1

您的真实用例可能不是SQLite，但如果是，您可以执行此类迁移而无需使用attached databases的Python往返：

from sqlalchemy import create_engine, MetaData, event
from sqlalchemy import Table, Column, Integer, String

metadata = MetaData()

person = Table('person', metadata,
               Column('id', Integer, primary_key=True),
               Column('name', String(250), nullable=False))

doppelganger = person.tometadata(metadata, schema='mirror')

def attach_mirror(dbapi_conn, connection_rec):
    dbapi_conn.execute("attach database 'mirror.db' as mirror")

engine = create_engine('sqlite:///source.db')
event.listen(engine, 'connect', attach_mirror)

metadata.create_all(engine)

with engine.connect() as conn:
    conn.execute(person.insert().values(name='James'))
    conn.execute(doppelganger.insert().prefix_with('OR IGNORE').
                 from_select(doppelganger.c, person.select()))

Answer 2

使用make_transient()删除所有会话信息，然后按预期添加。

from sqlalchemy.orm import make_transient

results = source_session.query(Person).all()
for row in results:
    mirror_session.add(make_transient(row))
mirror_session.commit()

如何在使用相同模型的不同数据库之间迁移数据？

2 个答案: