Question

当我在sqlalchemy中有多对多关系并且链接表被映射到一个类时，该ORM对象是如何以及何时创建的？我无法通过session.new找到它。例如。

这是一个解释它的最小例子：

from sqlalchemy import create_engine
from sqlalchemy import Column, DateTime, String, Integer, ForeignKey, func
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker


Base = declarative_base()


class Department(Base):
    __tablename__ = 'department'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    employees = relationship(
        'Employee',
        secondary='department_employee_link'
    )


class Employee(Base):
    __tablename__ = 'employee'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    hired_on = Column(DateTime, default=func.now())
    departments = relationship(
        Department,
        secondary='department_employee_link'
    )


class DepartmentEmployeeLink(Base):
    __tablename__ = 'department_employee_link'
    department_id = Column(Integer, ForeignKey('department.id'), primary_key=True)
    employee_id = Column(Integer, ForeignKey('employee.id'), primary_key=True)


engine = create_engine('sqlite:///:memory:')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)


session = Session()
department = Department(name='department')
employee = Employee(name='employee')
department.employees.append(employee)
session.add(department)

# When does DepartmentEmployeeLink get created if ever as an ORM object?
print session.new

谢谢！

Answer 1

除非您明确查询该表，否则不会创建DepartmentEmployeeLink的实例。要插入和查询Employee或Department个实例，DepartmentEmployeeLink 类对象中的信息就足够了。没有必要为此创建实例。

何时以及如何创建sqlalchemy链接表对象？

1 个答案: