postgresql - 在 sqlalchemy 中手动设置 id 时的竞争条件

Question

我希望我的所有对象都有一个由 PostgreSQL 设置的唯一 id，带有一个（序列）和另一个取决于第一个的 id。创建对象时，如果我在保存后设置第二个 id，我将在表上有一个 INSERT 和一个 UPDATE，这并不是最好的。所以只有一个 INSERT 我从 PostgreSQL 序列中获取 id 并用它设置 id 而不是让 PostgreSQL 在 INSERT 阶段执行它。我对 SQLAlchemy 很陌生，并且想确保这种做法是竞争条件证明。

感谢您对这个想法的想法

    class MyModel:
        def __init__(self, session, **data):
            """
            Base constructor for almost all model classes, performing common tasks
            """
            cls = type(self)
    
       
            if session:
                """To avoid having an UPDATE right after the INSERT we manually fetch
                the next available id using a postgresl internal
    
                SELECT nextval(pg_get_serial_sequence('events', 'id'));
    
                To do that we need the table's name and the sequence
                column's name, by chance we use the same name in all our
                model
                """
    
                table_name = cls.__tablename__
    
                qry = f"SELECT nextval(pg_get_serial_sequence('{table_name}', 'id'))"
    
                rs = session.execute(qry)
    
                # TODO : find a non ugly way to to that
                for row in rs:
                    next_id = row[0]
    
                # manually set the object id
                self.id = next_id
    
                # set the external_id before saving the object in the database
                self.ex_id = cls.ex_id_prefix + self.id
    
                session.add(self)
                session.flush([self])

标签： postgresqlsqlalchemyflask-sqlalchemy

Answer 1

如果您的目标是 Postgresql 12 或更高版本，您可以使用生成的列。SQLAlchemy 的Computed 列类型会创建这样一个列，我们可以传递一个 SQL 表达式来计算值。

该模型将如下所示：

class MyModel(Base):
    __tablename__ = 't68225046'

    ex_id_prefix = 'prefix_'

    id = sa.Column(sa.Integer, primary_key=True)
    ex_id = sa.Column(sa.String, 
                      sa.Computed(sa.text(":p || id::varchar").bindparams(p=ex_id_prefix)))

产生这个 DDL

CREATE TABLE t68225046 (
    id SERIAL NOT NULL, 
    ex_id VARCHAR GENERATED ALWAYS AS ('prefix_' || id::varchar) STORED, 
    PRIMARY KEY (id)
)

和一个插入语句

2021-09-19 ... INFO sqlalchemy.engine.Engine BEGIN (implicit)
2021-09-19 ... INFO sqlalchemy.engine.Engine INSERT INTO t68225046 DEFAULT VALUES RETURNING t68225046.id
2021-09-19 ... INFO sqlalchemy.engine.Engine [generated in 0.00014s] {}
2021-09-19 ... INFO sqlalchemy.engine.Engine COMMIT

对于 Postgresql 的早期版本，或者如果您不需要将值存储在数据库中，您可以使用混合属性来模拟它。

import sqlalchemy as sa
from sqlalchemy import orm
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.sql import cast


Base = orm.declarative_base()


class MyModel(Base):
    __tablename__ = 't68225046'

    ex_id_prefix = 'prefix_'

    id = sa.Column(sa.Integer, primary_key=True)

    @hybrid_property
    def ex_id(self):
        return self.ex_id_prefix + str(self.id)

    @ex_id.expression
    def ex_id(cls):
        # See https://stackoverflow.com/a/54487891/5320906
        return cls.ex_id_prefix + cast(cls.id, sa.String)