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| """ | |
| SQLAlchemy, PostgreSQL, and autocommit | |
| The goal: DB-level autocommit by default. No unnecessary transaction around reads. Explicit transactions (and nested transactions using savepoints) when desired. | |
| The below seems to achieve that, though I'm not very good at SQLAlchemy so it might be terrible. | |
| """ | |
| from contextlib import contextmanager | |
| from sqlalchemy import create_engine, event | |
| from sqlalchemy.orm import sessionmaker, Session as BaseSession | |
| class Session(BaseSession): | |
| def __init__(self, *a, **kw): | |
| super(Session, self).__init__(*a, **kw) | |
| self._in_atomic = False | |
| @contextmanager | |
| def atomic(self): | |
| """Transaction context manager. | |
| Will commit the transaction on successful completion of the block, or | |
| roll it back on error. | |
| Supports nested usage (via savepoints). | |
| """ | |
| outer = not self._in_atomic | |
| if outer: | |
| self.begin() | |
| self._in_atomic = True | |
| else: | |
| self.begin_nested() | |
| try: | |
| yield | |
| except: | |
| self.rollback() | |
| raise | |
| else: | |
| self.commit() | |
| finally: | |
| if outer: | |
| self._in_atomic = False | |
| def xadd(self, *objs): | |
| with self.atomic(): | |
| for obj in objs: | |
| self.add(obj) | |
| def is_deleted(self, obj): | |
| obj_id = obj.id | |
| self.expire(obj) | |
| return self.query(type(obj)).get(obj_id) is None | |
| class Database(object): | |
| def __init__(self, db_uri): | |
| self.engine = create_engine(db_uri, isolation_level="AUTOCOMMIT") | |
| self.Session = sessionmaker( | |
| bind=self.engine, class_=Session, autocommit=True) | |
| @event.listens_for(self.Session, 'after_begin') | |
| def receive_after_begin(session, transaction, connection): | |
| self.engine.dialect._set_connection_isolation( | |
| connection, 'READ COMMITTED') |
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