How to Create a Many-to-Many Relationship Database in Python Flask?

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To create a many-to-many relationship database in Python Flask, you can use SQLAlchemy, which is a popular Object-Relational Mapping (ORM) library. Here’s how you can create a many-to-many relationship between two tables in Flask:

Assume we have two tables: students and courses, and a many-to-many relationship between them. That is, a student can enroll in multiple courses, and a course can have multiple students.

First, we need to create a new table to represent the relationship between the two tables. This table is often referred to as a “junction” or “association” table. Here’s an example:

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

class Enrollment(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    student_id = db.Column(db.Integer, db.ForeignKey('students.id'))
    course_id = db.Column(db.Integer, db.ForeignKey('courses.id'))

In this example, we define a new class Enrollment that represents the junction table between the students and courses tables. The class has three columns: id, student_id, and course_id. The id column is a primary key, and the other two columns are foreign keys that refer to the id columns in the students and courses tables, respectively.

Next, we need to update the Student and Course classes to include a relationship to the Enrollment class. Here’s an example:

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    enrollments = db.relationship('Enrollment', backref='student', lazy=True)

class Course(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    enrollments = db.relationship('Enrollment', backref='course', lazy=True)

In this example, we add a new enrollments attribute to both the Student and Course classes, which represents the many-to-many relationship to the Enrollment class. We use the db.relationship function to define the relationship, with the backref parameter specifying the name of the attribute that will be added to the Enrollment class to represent the reverse relationship, and the lazy parameter specifying how the relationship should be loaded (in this case, lazily).

Finally, we can use these classes to query and manipulate the data in the database. For example, to add a new enrollment for a student and a course, we can do the following:

# create a new enrollment
enrollment = Enrollment(student_id=1, course_id=2)

# add the enrollment to the database session
db.session.add(enrollment)

# commit the changes to the database
db.session.commit()

This will create a new enrollment in the Enrollment table with the specified student_id and course_id. The db.session.add function adds the enrollment to the database session, and the db.session.commit function commits the changes to the database.

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