This schema fails as well as relationship definitions on both side with back_populates. Relationship definitions on both side without back_populates work fine. Exception is: sqlalchemy.orm.exc.FlushError: New instance <B at 0x2135e90> with identity key (<class '__main__.B'>, (1,)) conflicts with persistent instance <B at 0x212d850> Commenting th…

sa_merge_m2m_backref.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, ForeignKey, create_engine
from sqlalchemy.orm import sessionmaker, Session, relationship

Base = declarative_base()


class AB(Base):
    __tablename__ = 'AB'
    a_id = Column(ForeignKey('A.id'), primary_key=True)
    b_id = Column(ForeignKey('B.id'), primary_key=True)

class A(Base):
    __tablename__ = 'A'
    id = Column(Integer, primary_key=True)
    b = relationship('B', secondary=AB.__table__, backref='a')

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


engine = create_engine('sqlite://', echo=True)
Base.metadata.create_all(engine)
session = sessionmaker(bind=engine)()

b = B(id=1, a=[A(id=1)])
session.add(b)
session.commit()

b.a[0].b # <- Commenting this line changes exception to IntegrityError

b = B(id=1, a=[session.query(A).get(1)])
b = session.merge(b) # <- Fails
session.commit()

its not merge.

session = sessionmaker(bind=engine)()

b = B(id=1, a=[A(id=1)])
session.add(b)
session.commit()

b.a[0].b # <- Commenting this line changes exception to IntegrityError

# you just cascaded a brand new B(id=1) into the Session via backref
b = B(id=1, a=[session.query(A).get(1)])

# boom
session.flush()