如何在使用同一模型的不同数据库之间迁移数据？

Question

I would like to migrate some data between two databases that share the same model. Here is my code:

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()

The migration line mirror_session.add(row) produces the following error:

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

Answer 1

Your real use case might not be SQLite, but in case it is, you can perform such a migration without a round trip in Python with attached databases :

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

Use make_transient() to remove all session information, then you can add it as expected.

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()