SQLAlchemy - 查询多个表并返回嵌套对象

Question

Suppose we have a simple one-to-many relationship between Company and Employee, is there a way to query all companies and have a list of employees in the attribute of each company?

class Company(Base):
    __tablename__ = 'company'

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


class Employee(Base):
    __tablename__ = 'employee'

    id = Column(Integer, primary_key=True)
    first_name = Column(String)
    last_name = Column(String)
    company_id = Column(Integer, ForeignKey(Company.id))

I'm looking for something like this:

>>> result = db.session.query(Company).join(Employee).all()
>>> result[0].Employee
[<Employee object at 0x...>, <Employee object at 0x...>]

The size of result should be same as the number of rows in company table.

I tried the following and it gives tuple of objects (which makes sense) instead of nice parent / child structure:

>>> db.session.query(Company, Employee).filter(Company.id = Employee.company_id).all()

It's not hard to convert this into my desired object structure but just wanted to see if there's any shortcut.

Answer 1

You have to configure the relationship in the parent class:

class Company(Base):
    __tablename__ = 'company'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    employees = relationship('Employee', lazy='joined') # <<< Add this line

Then you can query it without a join:

companies = session.query(Company).all()
print(companies[0].employees)

Documentation: https://docs.sqlalchemy.org/en/13/orm/loading_relationships.html

Answer 2

You could do something like this:

class Company(Base):
    __tablename__ = 'company'

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

    employees = db.session.query(Company, Employee).filter(Company.id = self.id).all()
    self.employee_list = ['{0} {1}'.format(c.first_name, c.last_name) for c in employees]

Then you could access a employee name with Company.employee_list[0]