sqlalchemy/postgresql:数据库列计算的默认值 [英] sqlalchemy/postgresql: database column computed default value
问题描述
我有一个事务表,其中有tenant_id和transaction_id列(它们构成唯一的复合索引).对于插入操作,transaction_id必须增加1,但对于给定的tenant_id.因此,使用sqlalchemy框架,我手动找到了tenant_id的最大transaction_id:
I have a transaction table, which has tenant_id and transaction_id columns (they make up a unique composite index). For insert operation, transaction_id must be incremented by 1, but for given tenant_id. So, using sqlalchemy framework, I manually find max transaction_id for tenant_id:
res = db.session.query(func.max(my_tran.transaction_id).label('last_id')) \
.filter_by(tenant_id=tenant_id).one()
if res.last_id:
my_tran.transaction_id = res.last_id
else:
my_tran.transaction_id = 1
我想做的是将模型类的逻辑定义为服务器默认值:
What I'd like to do instead is define the logic for my model class as server default:
class MyTran(db.Model):
__tablename__ = 'my_tran'
id = db.Column(db.Integer, primary_key=True)
tenant_id = db.Column(db.Integer, db.ForeignKey('tenant.id'), nullable=False)
transaction_id = db.Column(db.Integer, nullable=False, \
server_default='compute last id for tenant_id + 1')
我想我需要创建一个触发器(如何?),但是不知道如何链接到我的模型类.
I guess I need to create a trigger (how?), but don't know how to link to my model class.
推荐答案
我找到了答案这里.详细信息如下(使用sqlalchemy):
I found my answer here. Details are below (using sqlalchemy):
create_fn_my_tran_set_num = DDL(
'''
CREATE OR REPLACE FUNCTION fn_my_tran_set_num()
RETURNS TRIGGER AS $$
DECLARE last_transaction_id INTEGER;
BEGIN
last_transaction_id := MAX(transaction_id) FROM my_tran WHERE tenant_id = NEW.tenant_id;
IF last_transaction_id IS NULL THEN
NEW.transaction_id := 1;
ELSE
NEW.transaction_id := last_transaction_id + 1;
END IF;
RETURN NEW;
END$$
LANGUAGE plpgsql
''')
event.listen(MyTran.__table__, 'after_create', create_fn_my_tran_set_trx_id)
create_tg_my_tran_set_num = DDL(
'''
CREATE TRIGGER tg_my_tran_set_num
BEFORE INSERT ON my_tran
FOR EACH ROW
EXECUTE PROCEDURE fn_my_tran_set_num();
''')
event.listen(MyTran.__table__, 'after_create', create_tg_my_tran_set_trx_id)
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