My Simple Guide to Database Relationships (From Confused to Clear 😅)

Heyyy guyyysss 🤭 (told you, you were gonna hear this a lot 😂)

I'm guessing you weren't expecting an article so soon, well I was working on a project in which I needed a table with a many-to-many relationship, then I realized, I don't fully understand how this relationship (database relationship ohh 😂) works.

So I’m gonna explain the 3 database relationships. Personally, I understand things better when I can visualize them, so I’ll be dropping the tabular representation right below each Django model. These tables show you how the database actually stores the information.

1. One-to-One (1️⃣ ↔ 1️⃣)

👉 Think of it like a person and their passport.

• Each person can only have one passport.

• Each passport belongs to one person only.

Example in Django:

class Person(models.Model):
    name = models.CharField(max_length=100)

class Passport(models.Model):
    number = models.CharField(max_length=20)
    person = models.OneToOneField(Person, on_delete=models.CASCADE)

Tabular Representation

Person Table

Passport Table

Note: The person column in the passport table stores a number that is a reference to the ID in the person table. E.g., the row that stores person as 1 is referring to Alice in the person table. We do not want to hardcode the person’s name into the field as this is bad practice, so in subsequent tables, you will be seeing the ID stored in the row.

📌Other usage includes:

• Profile + User

• Car + Engine

2. One-to-Many (1️⃣ ↔ 🌍) OR Many-to-One

👉 Think of it like a mother and her children.

• One mother can have many children.

• But each child has only one mother.

Example in Django:

class Mother(models.Model):
    name = models.CharField(max_length=100)

class Child(models.Model):
    name = models.CharField(max_length=100)
    mother = models.ForeignKey(Mother, on_delete=models.CASCADE)

Tabular representation

Mother Table

Child Table

📌Other usage includes:

• Company → Employees

• School → Student

3. Many-to-Many (🌍 ↔ 🌍)

👉 Think of it like students and courses.

• One student can enroll in many courses.

• One course can have many students.

Example in Django:

class Student(models.Model):
    name = models.CharField(max_length=100)
    courses = models.ManyToManyField("Course", through="Enrollment")

class Course(models.Model):
    title = models.CharField(max_length=100)

class Enrollment(models.Model):  # the "through table"
    student = models.ForeignKey(Student, on_delete=models.CASCADE)
    course = models.ForeignKey(Course, on_delete=models.CASCADE)
    date_enrolled = models.DateField()
    grade = models.CharField(max_length=5, blank=True)

Tabular Representation

Student Table

Course Table

Enrollment Table

📌Other usage includes:

• Doctors ↔ Patients

• Actors ↔ Movies

At first, many-to-many confused me because of the extra join table, but once I visualized it as a separate table (Enrollment), it clicked. Which of these relationships do you find most confusing in your projects? Drop a comment, let’s discuss 👇