Technical Lesson: Many-to-Many Relationships

Introduction

In this lesson, we will explore how to apply inheritance as well as class attributes and methods.

Scenario

Imagine you are working for a school administration. You want to keep track of all the students and all the courses. In this case, thinking about the relationship between the two, we can say that:

• A student has many courses
• A course has many students

This makes it a many-to-many relationship.

How can we connect the two?

We can use a schedule table to connect them.

• Create intermediary class
• Build relationships to allow access between classes
• Build properties to ensure class connection

Tools and Resources

• GitHub repo: python-many-to-many-technical-lesson

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Instructions

Set Up

Before we begin coding, let's complete the initial setup for this lesson:

Fork and Clone

• Go to the provided GitHub repository link.
• Fork the repository to your GitHub account.
• Clone the forked repository to your local machine.

Open and Run File

• Open the project in VSCode
• Run pipenv install to install all necessary dependencies
• Run pipenv shell to enter the virtual environment.

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Instructions

Task 1: Define the Problem

You need to build out a relationship between students and courses.

You will be tasked with:

• Creating an intermediary class
• Building relationships to allow access between classes
• Building properties to ensure class connection

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Task 2: Determine the Design

Schedule

Attributes:

• Student
• Course
• Grade

Properties:

• Student
• Course

Course

Attributes:

• Title
• Teacher

Methods:

• students

Student

Attributes:

• Name
• Email

Methods:

• courses
• calculate_gpa

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Task 3: Develop, Test, and Refine the Code

Step 1: Create a Feature Branch

git checkout -b many-to-many

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Step 2: Set up Intermediary Class

Ensure you understand Student and Course models. Then build the connecting Schedule class:

File: lib/many_to_many.py

class Student:
    all =[]
    def __init__(self, name, email):
        self.name = name
        self.email = email
        Student.all.append(self)

class Course:
    all =[]
    def __init__(self, title, teacher):
        self.title = title
        self.teacher = teacher
        Course.all.append(self)

class Schedule:
    all =[]
    def __init__(self, student, course, grade):
        self.student = student
        self.course = course
        self.grade = grade
        Schedule.all.append(self)

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Step 3: Set up Properties

Add properties to require that Schedule uses instances of Student and Course:

class Schedule:
    all =[]

    def __init__(self, student, course, grade):
        self.student = student
        self.course = course
        self.grade = grade
        Schedule.all.append(self)

    @property
    def student(self):
        return self._student

    @student.setter
    def student(self, value):
        if not isinstance(value, Student):
            raise Exception
        self._student = value

    @property
    def course(self):
        return self._course

    @course.setter
    def course(self, value):
        if not isinstance(value, Course):
            raise Exception
        self._course = value

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Step 4: Connecting Methods

Add methods to link Student ↔ Course through Schedule.

class Student:
    all =[]

    def __init__(self, name, email):
        self.name = name
        self.email = email
        Student.all.append(self)

    def courses(self):
        return [schedule.course for schedule in Schedule.all if schedule.student == self]

class Course:
    def __init__(self, title, teacher):
        self.title = title
        self.teacher = teacher
        Course.all.append(self)

    def students(self):
        return [schedule.student for schedule in Schedule.all if schedule.course == self]

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Step 5: Additional Methods

Build a calculate_gpa() method in Student:

class Student:
    ...
    def calculate_gpa(self):
        my_courses = self.courses()
        summative = 0
        for course in my_courses:
            summative += course.grade
        return summative / len(my_courses)

Use this to calculate a student's GPA based on course grades via Schedule.

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Step 6: Push Changes to GitHub and Merge Branches

git commit -am "Completed many to many models"
git push origin many-to-many

• Create a Pull Request (PR) on GitHub
• Merge the PR into main after review

Then, pull the updated main branch and clean up:

git checkout main
git pull origin main
git branch -d many-to-many
# If needed:
git branch -D many-to-many

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Task 4: Document and Maintain

Best Practices

• Add comments to explain code
• Clarify intent/functionality for others
• Add screenshots of completed work to README
• Update README text following makeareadme.com
• Delete stale branches
• Remove commented/unnecessary code
• Update .gitignore for sensitive data

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Considerations

1. Ternary

Ternaries are used to clean up for-loops. For example:

# Standard loop
all_students = []
for schedule in Schedule.all:
    if schedule.course == self:
        all_students.append(schedule.student)
return all_students

# Same as:
return [schedule.student for schedule in Schedule.all if schedule.course == self]

2. Ensuring Class Instances

You can treat your own classes the same way you check for int or str using isinstance():

if not isinstance(value, Student):
    raise Exception

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