Sqlalchemy 简明教程

Many to Many Relationships

两个表之间的 Many to Many relationship 通过添加一个关联表来实现，关联表有两个外键 - 一个来自每个表的主键。此外，映射到两个表的类具有一个属性，该属性具有一个集合对象的其他关联表被指定为 relationship() 函数的辅助属性。

为此，我们将在 SQLite 数据库 (mycollege.db) 中创建两个表 - 部门和员工。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员工类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')

现对 Link 类执行定义。其与关联表相连，并且分别包含分别引用部门键和员工表的主键的 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)

在此，我们必须注意，Department 类具有与 Employee 类相关的 employee 属性。关联功能的次要属性被分配为链接作为其值。

相似地，Employee 类具有与 Department 类相关的 department 属性。关联功能的次要属性被分配为链接作为其值。

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 来检查这一点，如下所示：

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。相似地，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)

接下来，我们需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修改，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()

在 Python 控制台中会发出以下 SQL 语句：

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))

要检查上述操作的影响，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，输出将如下所述显示：

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