如何从 sqlalchemy 中的上层访问子类？答案

【问题标题】：How can I access subclasses from upper class in sqlalchemy?如何从 sqlalchemy 中的上层访问子类？
【发布时间】：2020-11-07 15:45:08
【问题描述】：

我有 3 节课；

'Company' 顶级类它的子类'Department' 它的子类'DepartmentalUnit'
我可以访问从'DepartmentalUnit'类到顶级'Company'所有类的值

尽管阅读了文档，但我无法做到和理解的是；;

我无法访问“公司”类的其他子类

class Company(Base):
    __tablename__ = 'company'

    id = Column(Integer, primary_key=True)
    name = Column(String)

    departments = relationship('Department',backref='company')


class Department(Base):
    __tablename__ = 'department'

    id = Column(Integer, primary_key=True)
    department_name = Column(String)

    company_id = Column(Integer, ForeignKey('company.id'))
    departmentalunits = relationship('DepartmentalUnit', backref='department')


class DepartmentalUnit(Base):

    __tablename__ = 'departmentalunit'

    id = Column(Integer, primary_key=True,nullable=False)
    departmental_unit_name = Column(String)

    departments_id = Column(Integer, ForeignKey('department.id'))

我从子类访问上层类的代码：

query = session.query(DepartmentalUnit)
instance = query.all()

for i in instance:
    print(i.department.company.name)
    print(i.department.department_name)
    print(i.departmental_unit_name)

无法从公司类访问其他子类的代码：

query = session.query(Company)
instance = query.all()

for i in instance:
    print(i.department.department_name)

【问题讨论】：

标签： python sqlalchemy

【解决方案1】：

您的最后一个查询应该以不同的方式使用：

关系名称中有错字：应该是departments 而不是department
鉴于关系是 1-N，结果是一个列表，因此您应该遍历子级。

这应该可行：

query = session.query(Company)

for company in query.all():
    print(company.name)
    for dep in company.departments:
        print("  ", dep.department_name)
        for dep_unit in dep.departmentalunits:
            print("    ", dep_unit.departmental_unit_name)

【讨论】：

我通过在关系中添加 backref 解决了这个问题。非常感谢。

【解决方案2】：

我解决了这个问题。我为关系添加了一个反向引用，现在我可以从公司访问所有这些。不确定这是否是正确的方法？但是，我目前正在获得我想要的回报。我还没有未答复的请求。

示例已解决：

class Company(Base):
    __tablename__ = 'company'

    id = Column(Integer, primary_key=True)
    name = Column(String)

    departments = relationship('Department',backref='company',uselist=False)


class Department(Base):
    __tablename__ = 'department'

    id = Column(Integer, primary_key=True)
    department_name = Column(String)

    company_id = Column(Integer, ForeignKey('company.id'))
    departmentalunits = relationship('DepartmentalUnit', backref='department',uselist=False)

class DepartmentalUnit(Base):

    __tablename__ = 'departmentalunit'

    id = Column(Integer, primary_key=True,nullable=False)
    departmental_unit_name = Column(String)

    departments_id = Column(Integer, ForeignKey('department.id'))


query = session.query(Company)
instance = query.all()

for i in instance:
    print(f"Company: {i.name}")

    print(f"Department: {i.departments.department_name}")

    print(f"Department Unit: {i.departments.departmentalunits.departmental_unit_name}")

    print( f"Report Category : {i.departments.departmentalunits.reportcategoryoftheunit.report_category_name}")

【讨论】：