我有一个需要使用upsert（插入，如果存在则更新）功能的多进程应用程序。
我决定使用触发器解决upsert问题。（您为每个启用upsert的表添加名为is_upsert的附加列，并在触发器中检查此字段。如果为false，则执行正常插入操作；但如果为true，则执行upsert逻辑-尝试更新，如果因记录不存在而失败，则尝试插入）。
这里是触发器逻辑：
CREATE OR REPLACE FUNCTION upsert_trigger_function_{table}()
RETURNS TRIGGER AS $upsert_trigger_function$
DECLARE
row record;
BEGIN
RAISE NOTICE 'upsert trigger fired, upsert is %%', NEW.{upsert_column};
IF NEW.{upsert_column} THEN
NEW.{upsert_column} := false;
LOOP
UPDATE {table} SET
{update_set}
WHERE
{update_where}
;
IF found THEN
RETURN NULL;
END IF;
BEGIN
INSERT INTO {table} SELECT NEW.*;
RETURN NULL;
EXCEPTION WHEN unique_violation THEN
-- loop
END;
END LOOP;
RETURN NULL;
ELSE
RETURN NEW;
END IF;
END;
$upsert_trigger_function$ LANGUAGE plpgsql;
测试对象，（add_upsert 只是使上面的触发器被安装）：
class SimpleItem(PipelinesBase):
__tablename__ = 'simple_item'
id = Column(BigInteger, primary_key=True)
item_type = Column(String, nullable=False, unique=True)
quantity = Column(Integer, nullable=False)
price = Column(Float, nullable=False)
in_stock = Column(Boolean, nullable=False)
arrived = Column(Date)
sys_time = Column(
TSTZRANGE,
nullable=False,
server_default=text("TSTZRANGE(now(), null)"),
)
_upsert = Column(Boolean, nullable=False, server_default=text('false'))
_type_identifier = 1400
add_upsert(SimpleItem, ['item_type'])
测试脚本
from sqlalchemy.engine import create_engine
from pipelines.settings_proxy import TEST_DB
from sqlalchemy.orm.session import sessionmaker
from test_pipelines.test_persistence.mock_items import SimpleItem
from test_pipelines.test_persistence.helpers import random_simple_item
def main():
engine = create_engine(TEST_DB)
values = random_simple_item(_upsert=True)
session = sessionmaker(engine)()
si = SimpleItem(**values)
session.add(si)
session.commit()
si = SimpleItem(**values)
si.price = 1
session.merge(si)
session.commit()
使用SQL语句时它能正常工作，但是当我将其与SQLAlchemy ORM添加对象一起使用时，会出现问题。
Traceback (most recent call last):
File "pipelines/persistence/experiment_with_upsert_field.py", line 59, in <module>
main()
File "pipelines/persistence/experiment_with_upsert_field.py", line 27, in main
session.commit()
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 801, in commit
self.transaction.commit()
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 392, in commit
self._prepare_impl()
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 372, in _prepare_impl
self.session.flush()
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 2019, in flush
self._flush(objects)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 2137, in _flush
transaction.rollback(_capture_exception=True)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__
compat.reraise(exc_type, exc_value, exc_tb)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/util/compat.py", line 184, in reraise
raise value
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 2101, in _flush
flush_context.execute()
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/unitofwork.py", line 373, in execute
rec.execute(self)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/unitofwork.py", line 532, in execute
uow
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/persistence.py", line 174, in save_obj
mapper, table, insert)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/orm/persistence.py", line 800, in _emit_insert_statements
execute(statement, params)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 914, in execute
return meth(self, multiparams, params)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
return connection._execute_clauseelement(self, multiparams, params)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
compiled_sql, distilled_params
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 1159, in _execute_context
result = context._setup_crud_result_proxy()
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/engine/default.py", line 828, in _setup_crud_result_proxy
self._setup_ins_pk_from_implicit_returning(row)
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/engine/default.py", line 893, in _setup_ins_pk_from_implicit_returning
for col in table.primary_key
File "/home/sebastian/local/virtualenvs/perception/lib/python3.4/site-packages/sqlalchemy/engine/default.py", line 891, in <listcomp>
for col, value in [
TypeError: 'NoneType' object is not subscriptable
在sqlalchemy.engine.default的深处引发错误。我相信这是因为我的触发器在执行UPSERT时返回NULL，而SQLAlchemy试图使用RETURNING语句传播具有插入ID的对象。显然会失败，因为无法从下属的INSERT / UPDATE中获取正确的ID，同时阻止正常的插入。
请注意，我已经测试了upsert作为特殊函数，但对于更新与其他项目存在关系（具有关系）的复杂项目，它对我没有用。
所以我的问题是：如何告诉SQLAlchemy避免加载插入对象的ID？