在sqlalchemy中从关系中获取对象的属性(Get attribute of object from relationship in sqlalchemy)

 请考虑以下定义某些类的代码： 
 
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, Float, String, DateTime
from sqlalchemy import ForeignKey, UniqueConstraint
from sqlalchemy.orm import relationship, backref

class Base(object):
    id = Column(Integer, primary_key=True)
Base = declarative_base(cls=Base)

class Teacher(Base):
    __tablename__ = 'teacher'

    email = Column(String, unique=True)
    name = Column(String, nullable=False)
    password = Column(String)
    base_score = Column(Float)
    time_zone = Column(String)

    def __repr__(self):
        return '<Teacher({0})>'.format(self.email)

class Class(Base):
    __tablename__ = 'class'

    teacher_id = Column(Integer, ForeignKey('teacher.id', ondelete="CASCADE"), nullable=False, index=True)
    Teacher = relationship("Teacher", backref=backref('classes', order_by='Class.id'))
    name = Column(String, index=True)

    __table_args__ = (UniqueConstraint('teacher_id','name'),)

    def __repr__(self):
        return '<Class({0})>'.format(self.name)

class Student(Base):
    __tablename__ = 'student'

    class_id = Column(Integer, ForeignKey('class.id', ondelete="CASCADE"), nullable=False, index=True)
    Class = relationship("Class", backref=backref('students', order_by='Student.id'))
    name = Column(String, index=True)

    __table_args__ = (UniqueConstraint('class_id','name'),)

    def __repr__(self):
        return '<Student({0})>'.format(self.name)
 
 以下代码使用类： 
 
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

engine = create_engine('sqlite:///:memory:', echo=True)

Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

t = Teacher(email='test@example.com',name='Cool Guy',base_score=100,time_zone='America/Chicago')

c1 = Class(Teacher=t,name='Mathematics')
s1 = Student(Class=c1,name='Bob Roberts')
 
 首先，我想知道是否正确定义c1是正确的方法（使用Teacher参数。） 
 
 其次，我知道我是否可以通过以下方式获得学生的老师： 
 
s1.Class.Teacher
 
 但是，无论如何（并且正常）在Student类上直接创建Teacher属性？ 我尝试添加： 
 
Teacher = Class.Teacher
 
 在类定义中，但是扔了： 
 
AttributeError: 'RelationshipProperty' object has no attribute 'Teacher'
 
 显然，这不是什么大不了的事，但我想知道正确的方式（如果有一个，或者没有一个），因为我正在努力学习sqlalchemy。 

Consider the following code defining some classes:
 
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, Float, String, DateTime
from sqlalchemy import ForeignKey, UniqueConstraint
from sqlalchemy.orm import relationship, backref

class Base(object):
    id = Column(Integer, primary_key=True)
Base = declarative_base(cls=Base)

class Teacher(Base):
    __tablename__ = 'teacher'

    email = Column(String, unique=True)
    name = Column(String, nullable=False)
    password = Column(String)
    base_score = Column(Float)
    time_zone = Column(String)

    def __repr__(self):
        return '<Teacher({0})>'.format(self.email)

class Class(Base):
    __tablename__ = 'class'

    teacher_id = Column(Integer, ForeignKey('teacher.id', ondelete="CASCADE"), nullable=False, index=True)
    Teacher = relationship("Teacher", backref=backref('classes', order_by='Class.id'))
    name = Column(String, index=True)

    __table_args__ = (UniqueConstraint('teacher_id','name'),)

    def __repr__(self):
        return '<Class({0})>'.format(self.name)

class Student(Base):
    __tablename__ = 'student'

    class_id = Column(Integer, ForeignKey('class.id', ondelete="CASCADE"), nullable=False, index=True)
    Class = relationship("Class", backref=backref('students', order_by='Student.id'))
    name = Column(String, index=True)

    __table_args__ = (UniqueConstraint('class_id','name'),)

    def __repr__(self):
        return '<Student({0})>'.format(self.name)
 
And the following code using the classes:
 
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

engine = create_engine('sqlite:///:memory:', echo=True)

Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

t = Teacher(email='test@example.com',name='Cool Guy',base_score=100,time_zone='America/Chicago')

c1 = Class(Teacher=t,name='Mathematics')
s1 = Student(Class=c1,name='Bob Roberts')
 
First of all, I want to know if defining c1 like that is the right way (Using the Teacher argument.)
 
Second of all, I know if I can get the Teacher of a student by doing:
 
s1.Class.Teacher
 
But is there anyway ( and is it normal) to create a Teacher attribute directly on the Student class? I tried adding:
 
Teacher = Class.Teacher
 
In the class definition, but that threw:
 
AttributeError: 'RelationshipProperty' object has no attribute 'Teacher'
 
Obviously, it's not that big of a deal, but I want to know the correct way (if there is one, or that there is not one) as I'm trying to learn sqlalchemy better.

原文：https://stackoverflow.com/questions/16391768