需要经典的映射器示例以实现SqlAlchemy单表继承 [英] Need classic mapper example for SqlAlchemy single table inheritance

问题描述

我找到了一个有关如何使用类映射进行单表继承的示例.

I found an example of how to do single table inheritance using Class mappings.

http://docs.sqlalchemy.org /en/latest/orm/inheritance.html#single-table-inheritance

但是对于我一生来说，我找不到如何使用经典映射器执行此操作的示例，这样我就可以使类和持久性映射保持独立.

But for the life of me, I cannot find an example of how to do this with classic mapper so that I can keep my classes and persistent mappings separate.

如何将本示例转换为经典映射?我很清楚创建表的方式，只是不确定如何实际构造映射器.

How do I convert this example into classic mapping? I am clear on creating the tables, just not sure how to actually structure the mapper.

在示例中，定义了以下类型:

In the example, there are the following types defined:

class Employee(Base):

class Manager(Employee):

class Engineer(Employee):

假设我已经创建了适当的表:

Assuming I have created the appropriate table:

employee = Table(...Column(type...))

如何为映射器编写代码，以便Manager和Engineer都生活在按类型(经理"，工程师"或其他雇员)区分的同一张表中(单表继承)?

How do I write code for the mapper so that both Manager and Engineer live in the same table (single table inheritance) discriminated by type ("manager", "engineer" or otherwise employee)?

谢谢.

推荐答案

手动映射类继承层次结构很费力，不是我所建议的，但这是可行的.首先定义表.由于使用单表继承，因此它必须包括所有必需的列:

Manually mapping class inheritance hierarchies is laborious and not something I'd recommend, but here goes. Start by defining your table. Since using single table inheritance, it must include all the required columns:

metadata = MetaData()

employee = Table(
    'employee',
    metadata,
    Column('id', Integer, primary_key=True),
    Column('name', String(50)),
    Column('type', String(20)),
    Column('manager_data', String(50)),
    Column('engineer_info', String(50))
)

普通的Python类:

The plain Python classes:

class Employee:

    def __init__(self, name):
        self.name = name

class Manager(Employee):

    def __init__(self, name, manager_data):
        super().__init__(name)
        self.manager_data = manager_data

class Engineer(Employee):

    def __init__(self, name, engineer_info):
        super().__init__(name)
        self.engineer_info = engineer_info

以及经典映射:

mapper(Employee, employee,
       polymorphic_on=employee.c.type,
       polymorphic_identity='employee',
       exclude_properties={'engineer_info', 'manager_data'})


mapper(Manager,
       inherits=Employee,
       polymorphic_identity='manager',
       exclude_properties={'engineer_info'})


mapper(Engineer,
       inherits=Employee,
       polymorphic_identity='engineer',
       exclude_properties={'manager_data'})

请注意您必须如何手动限制每个映射器中的映射属性，这对于较大的层次结构将变得难以维护.使用声明式时，为您处理的所有事情.

Note how you have to limit the mapped properties manually in each mapper, which will become hard to maintain with larger hierarchies. When using Declarative all that is handled for you.

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