SQLAlchemy Obtain Primary Key With Autoincrement Before Commit

Question

When I have created a table with an auto-incrementing primary key, is there a way to obtain what the primary key would be (that is, do something like reserve the primary key) without actually committing?

I would like to place two operations inside a transaction however one of the operations will depend on what primary key was assigned in the previous operation.

Answer 1

You can use multiple transactions and manage it within scope.

Answer 2

I don't know anything about SQL Alchemy.. but if the database is SQl Server then you simply do your insert statement and record the value of @@IDENTITY... example:

DECLARE @ID BIGINT

BEGIN TRANSACTION INSERT INTO table (col1, col2, col3) VALUES (val1, val2, val3) SET @ID = @@IDENTITY

INSERT INTO table2 (Fk1, col1, col2) VALUES (@ID, val1, val2)

COMMIT TRANSACTION

Answer 3

You don't need to commit, you just need to flush. Here's some sample code. After the call to flush you can access the primary key that was assigned. Note this is with SA 0.4.8.

from sqlalchemy import *
from sqlalchemy.databases.mysql import *
import sqlalchemy.ext.declarative

Base = sqlalchemy.ext.declarative.declarative_base()

class User(Base):
    __tablename__ = 'user'
    user_id = Column('user_id', Integer, primary_key=True)
    name = Column('name', String)

if __name__ == '__main__':
    import unittest
    from sqlalchemy.orm import *
    import datetime

    class Blah(unittest.TestCase):
        def setUp(self):
            self.engine = create_engine('sqlite:///:memory:', echo=True)
            self.sessionmaker = scoped_session(sessionmaker(bind=self.engine))
            Base.metadata.bind = self.engine
            Base.metadata.create_all()
            self.now = datetime.datetime.now()

        def test_pkid(self):
            user = User(name="Joe")
            session = self.sessionmaker()
            session.save(user)
            session.flush()
            print 'user_id', user.user_id
            session.commit()
            session.close()

    unittest.main()