What is the difference between ForeignKey, OneToOneField, and ManyToManyField in Django models? Django models में ForeignKey, OneToOneField, और ManyToManyField में क्या अंतर है?

from django.db import models
from django.contrib.auth.models import User

# ForeignKey - many books can belong to one author
class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    title = models.CharField(max_length=200)
    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books')

author = Author.objects.get(id=1)
author.books.all()  # access related books via related_name

# OneToOneField - each User has exactly one Profile
class Profile(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    bio = models.TextField(blank=True)

user = User.objects.get(id=1)
profile = user.profile  # direct one-to-one access

# ManyToManyField - students can enroll in many courses, courses can
# have many students; Django creates a hidden junction table automatically
class Course(models.Model):
    name = models.CharField(max_length=100)

class Student(models.Model):
    name = models.CharField(max_length=100)
    courses = models.ManyToManyField(Course, related_name='students')

student = Student.objects.get(id=1)
student.courses.add(course1, course2)  # adding relationships
student.courses.all()  # all courses this student is enrolled in

course = Course.objects.get(id=1)
course.students.all()  # all students enrolled in this course