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📜  多对多关系

📅  最后修改于: 2020-11-27 07:55:40             🧑  作者: Mango

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，该关联表具有两个外键-每个表的主键中的一个。此外，映射到两个表的类具有一个属性，该属性具有其他关联表的对象的集合，这些对象被分配为Relationship()函数的辅助属性。

为此，我们将创建一个带有两个表(部门和员工)的SQLite数据库(mycollege.db)。在这里，我们假设一个雇员是一个以上部门的一部分，而一个部门有一个以上雇员。这构成了多对多关系。

映射到部门和雇员表的雇员和部门类的定义如下-

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')

现在，我们定义一个Link类。它链接到链接表，并包含分别引用部门表和雇员表的主键的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)

在这里，我们必须记下Department类具有与Employee类相关的employee属性。关系函数的辅助属性被分配一个链接作为其值。

同样，雇员类具有与部门类相关的部门属性。关系函数的辅助属性被分配一个链接作为其值。

当执行以下语句时，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进行检查，如下面的屏幕截图所示-

部门表

员工表

链接表

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collection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department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，向其中添加所有对象，然后提交更改，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-

部门表数据

员工表数据

链接表数据

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