Sqlalchemy get_or_create implement

gistfile1.py

# -*- coding: utf-8 -*-

from sqlalchemy.exc import IntegrityError
from sqlalchemy.sql.expression import ClauseElement

def _get_or_create(session, model, defaults=None, **kwargs):
    try:
        query = session.query(model).filter_by(**kwargs)

        instance = query.first()

        if instance:
            return instance, False
        else:
            session.begin(nested=True)
            try:
                params = dict((k, v) for k, v in kwargs.iteritems() if not isinstance(v, ClauseElement))
                params.update(defaults)
                instance = model(**params)

                session.add(instance)
                session.commit()

                return instance, True
            except IntegrityError as e:
                session.rollback()
                instance = query.one()

                return instance, False
    except Exception as e:
        raise e

Note that this avoids one race condition (a create that happens between the query on line 10 and the create on line 22 will be caught on line 25), but introduces another: if the newly-created record that raises the IntegrityError is deleted between line 26 and line 27, then line 27 will raise NoResultFound.

I do not have a helpful solution for this problem; race conditions are tricky beasts.