Race condition when manually setting the id in sqlalchemy - postgresql

Question

I want that all my object have a unique id that is set by PostgreSQL with a (serial) and another id that depends to the first one.
When creating an object if I set the second id after saving it, I'll have an INSERT and an UPDATE on the table, what is not really the best.
So to have only one INSERT I fetch the id from the PostgreSQL sequence and set the id with it instead of letting PostgreSQL do it at INSERT stage.
I'm pretty new on SQLAlchemy and want to be sure that this way of doing is race condition proof.
Thanks for you thoughts on this idea
class MyModel:
def __init__(self, session, **data):
"""
Base constructor for almost all model classes, performing common tasks
"""
cls = type(self)
if session:
"""To avoid having an UPDATE right after the INSERT we manually fetch
the next available id using a postgresl internal
SELECT nextval(pg_get_serial_sequence('events', 'id'));
To do that we need the table's name and the sequence
column's name, by chance we use the same name in all our
model
"""
table_name = cls.__tablename__
qry = f"SELECT nextval(pg_get_serial_sequence('{table_name}', 'id'))"
rs = session.execute(qry)
# TODO : find a non ugly way to to that
for row in rs:
next_id = row[0]
# manually set the object id
self.id = next_id
# set the external_id before saving the object in the database
self.ex_id = cls.ex_id_prefix + self.id
session.add(self)
session.flush([self])

Answer 1

If you are targetting Postgresql 12 or later, you can use a generated column. SQLAlchemy's Computed column type will create such a column, and we can pass an SQL expression to compute the value.
The model would look like this:
class MyModel(Base):
__tablename__ = 't68225046'
ex_id_prefix = 'prefix_'
id = sa.Column(sa.Integer, primary_key=True)
ex_id = sa.Column(sa.String,
sa.Computed(sa.text(":p || id::varchar").bindparams(p=ex_id_prefix)))
producing this DDL
CREATE TABLE t68225046 (
id SERIAL NOT NULL,
ex_id VARCHAR GENERATED ALWAYS AS ('prefix_' || id::varchar) STORED,
PRIMARY KEY (id)
)
and a single insert statement
2021-09-19 ... INFO sqlalchemy.engine.Engine BEGIN (implicit)
2021-09-19 ... INFO sqlalchemy.engine.Engine INSERT INTO t68225046 DEFAULT VALUES RETURNING t68225046.id
2021-09-19 ... INFO sqlalchemy.engine.Engine [generated in 0.00014s] {}
2021-09-19 ... INFO sqlalchemy.engine.Engine COMMIT
For earlier releases of Postgresql, or if you don't need to store the value in the database, you could simulate it with a hybrid property.
import sqlalchemy as sa
from sqlalchemy import orm
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.sql import cast
Base = orm.declarative_base()
class MyModel(Base):
__tablename__ = 't68225046'
ex_id_prefix = 'prefix_'
id = sa.Column(sa.Integer, primary_key=True)
#hybrid_property
def ex_id(self):
return self.ex_id_prefix + str(self.id)
#ex_id.expression
def ex_id(cls):
# See https://stackoverflow.com/a/54487891/5320906
return cls.ex_id_prefix + cast(cls.id, sa.String)