Dear Michael and co,
I think that Columns which are marked as server_default=FetchedValue(), which
are normally omitted from INSERT statements, are not omitted after an object
has been merged from another session. Therefore SQLAlchemy tries to assign the
old values to them, which fails if the database does not allow it (e.g.
internal columns, such as xmin in Postgres).
If this is by design, is there any way to override it on the individual objects
before merging?
Here is an example that reproduces the error:
from sqlalchemy import Column, FetchedValue, Integer, Text, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import object_session, sessionmaker
Base = declarative_base()
class Dog(Base):
__tablename__ = 'dog'
__table_args__ = {'prefixes': ['TEMPORARY']}
id = Column(Integer, primary_key=True)
xmin = Column("xmin", Integer, server_default=FetchedValue(),
server_onupdate=FetchedValue(), system=True)
name = Column(Text)
def main():
engine = create_engine('postgresql://username@server:port/db')
engine.echo = True
Base.metadata.create_all(engine)
session_1 = sessionmaker(bind=engine)(autocommit=True)
with session_1.begin():
fido_1 = Dog(name="fido")
session_1.add(fido_1)
print(fido_1.xmin)
session_2 = sessionmaker(bind=engine)(autocommit=True)
with session_2.begin():
session_2.query(Dog).delete()
with sessionmaker(bind=engine)(autocommit=True).begin():
fido_2 = session_2.merge(fido_1)
session_2.add(fido_2)
print(fido_2.xmin)
print(session_2.query(Dog).count())
if __name__ == '__main__':
main()
And the result is:
sqlalchemy.exc.ProgrammingError: (raised as a result of Query-invoked
autoflush; consider using a session.no_autoflush block if this flush is
occurring prematurely)
(psycopg2.ProgrammingError) column "xmin" of relation "dog" does not exist
LINE 1: INSERT INTO dog (id, xmin, name) VALUES (1, '283302364', 'fi...
^
[SQL: INSERT INTO dog (id, xmin, name) VALUES (%(id)s, %(xmin)s, %(name)s)]
[parameters: {'id': 1, 'xmin': '283302364', 'name': 'fido'}]
(Background on this error at: http://sqlalche.me/e/f405)
Note that this is raised when fido_2 is inserted (after fido_1 is merged into a
new session).
Thanks, Chris.
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