多对多关系

两个表之间的多对多关系是通过添加一个关联表来实现的，该关联表有两个外键 - 一个来自每个表的主键。此外，映射到这两个表的类具有一个属性，该属性包含其他关联表的对象集合，该属性被指定为 relationship() 函数的次要属性。

为此，我们将创建一个包含两个表（部门和员工）的 SQLite 数据库 (mycollege.db)。在这里，我们假设一个员工属于多个部门，并且一个部门有多个员工。这构成了多对多关系。

映射到部门和员工表的 Employee 和 Department 类的定义如下 −

from sqlalchemy import create_engine, ForeignKey, Column, Integer, String
engine = create_engine('sqlite:///mycollege.db', echo = True)
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
from sqlalchemy.orm import relationship

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

我们现在定义一个 Link 类。它链接到链接表，并包含分别引用部门和员工表主键的 Department_id 和 employee_id 属性。

class Link(Base):
   __tablename__ = 'link'
   department_id = Column(
      Integer, 
      ForeignKey('department.id'), 
      primary_key = True)

employee_id = Column(
   Integer, 
   ForeignKey('employee.id'), 
   primary_key = True)

这里，我们必须注意，Department 类具有与 Employee 类相关的 employees 属性。关系函数的次要属性被分配一个链接作为其值。

同样，Employee 类具有与 Department 类相关的 Departments 属性。关系函数的次要属性被分配一个链接作为其值。

执行以下语句 − 时，将创建所有这三个表

Base.metadata.create_all(engine)

Python 控制台发出以下 CREATE TABLE 查询 −

CREATE TABLE department (
   id INTEGER NOT NULL,
   name VARCHAR,
   PRIMARY KEY (id)
)

CREATE TABLE employee (
   id INTEGER NOT NULL,
   name VARCHAR,
   PRIMARY KEY (id)
)

CREATE TABLE link (
   department_id INTEGER NOT NULL,
   employee_id INTEGER NOT NULL,
   PRIMARY KEY (department_id, employee_id),
   FOREIGN KEY(department_id) REFERENCES department (id),
   FOREIGN KEY(employee_id) REFERENCES employee (id)
)

我们可以通过使用 SQLiteStudio 打开 mycollege.db 来检查这一点，如下面的屏幕截图所示 −

接下来，我们创建 Department 类的三个对象和 Employee 类的三个对象，如下所示 −

d1 = Department(name = "Accounts")
d2 = Department(name = "Sales")
d3 = Department(name = "Marketing")

e1 = Employee(name = "John")
e2 = Employee(name = "Tony")
e3 = Employee(name = "Graham")

每个表都有一个具有 append() 方法的集合属性。我们可以将 Employee 对象添加到 Department 对象的 Employees 集合中。同样，我们可以将 Department 对象添加到 Employee 对象的 Departments 集合属性中。

e1.departments.append(d1)
e2.departments.append(d3)
d1.employees.append(e3)
d2.employees.append(e2)
d3.employees.append(e1)
e3.departments.append(d2)

我们现在要做的就是设置一个会话对象，将所有对象添加到其中并提交更改，如下所示 −

from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind = engine)
session = Session()
session.add(e1)
session.add(e2)
session.add(d1)
session.add(d2)
session.add(d3)
session.add(e3)
session.commit()

以下 SQL 语句将在 Python 控制台上发出 −

INSERT INTO department (name) VALUES (?) ('Accounts',)
INSERT INTO department (name) VALUES (?) ('Sales',)
INSERT INTO department (name) VALUES (?) ('Marketing',)
INSERT INTO employee (name) VALUES (?) ('John',)
INSERT INTO employee (name) VALUES (?) ('Graham',)
INSERT INTO employee (name) VALUES (?) ('Tony',)
INSERT INTO link (department_id, employee_id) VALUES (?, ?) ((1, 2), (3, 1), (2, 3))
INSERT INTO link (department_id, employee_id) VALUES (?, ?) ((1, 1), (2, 2), (3, 3))

要检查上述操作的效果，请使用 SQLiteStudio 并查看部门、员工和链接表中的数据 −

要显示数据，请运行以下查询语句 −

from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind = engine)
session = Session()

for x in session.query( Department, Employee).filter(Link.department_id == Department.id,
    Link.employee_id == Employee.id).order_by(Link.department_id).all():
    print ("Department: {} Name: {}".format(x.Department.name, x.Employee.name))

根据我们示例中填充的数据，输出将显示如下−

Department: Accounts Name: John
Department: Accounts Name: Graham
Department: Sales Name: Graham
Department: Sales Name: Tony
Department: Marketing Name: John
Department: Marketing Name: Tony