Flask: Develop Web Applications in Python/

...

/

The Many-to-Many Relationship

The Many-to-Many Relationship

In this lesson, we will learn how to add a many-to-many relationship between models.

We'll cover the following...

Introduction #

In the last lessons, we created both a one-to-many and a one-to-one relationship between the Employee and Department models. The code so far is given below.

Python 3.5
class Employee(db.Model):
    employee_id = db.Column(db.Integer, primary_key = True)
    first_name = db.Column(db.String(50), nullable = False)
    last_name = db.Column(db.String(50), nullable = False)
    department_name = db.Column(db.String, db.ForeignKey('department.name'), nullable = False)
    is_head_of = db.Column(db.String, db.ForeignKey('department.name'), nullable=True)
        
class Department(db.Model):
    name = db.Column(db.String(50), primary_key=True, nullable=False)
    location = db.Column(db.String(120), nullable=False)
    employees = db.relationship('Employee', backref='department')
    head = db.relationship('Employee', backref='head_of_department', uselist=False)
    
class Project(db.Model):
    project_id = db.Column(db.Integer, primary_key=True, nullable=False)
    name = db.Column(db.String(100), nullable=False)

Now, let’s find out how to add a many-to-many relationship between models.

Representing a many-to-many relationship in models #

In the example, a many-to-many relationship should exist between the Employee and the Project. This relationship will indicate that an employee can work on multiple projects while a project can have multiple employees working on it at the same time. ...