Database relationships in Django

Short visual memo for the three ways to define database relationships in Django.

Many-to-One

class Employee(models.Model):
    department = models.ForeignKey(Department, on_delete=models.CASCADE)

• each Employee only has one Department
• but a Department has multiple Employees

employee.department.pk
department.employee_set.all()

Many-to-many

class Employee(models.Model):
    projects = models.ManyToManyField(Project)

• each Employee works on multiple Projects
• each Project can be worked on by multiple Employees

employee.projects.all()
projects.employee_set.all()

To query the intermediary table:

employee.projects.through.objects.all()
projects.employee_set.through.objects.all()

One-to-one

class Employee(models.Model):
    resume = models.OneToOneField(Resume, on_delete=models.CASCADE)

• each Employee only has one Resume
• each Resume is owned by only one Employee

employee.resume.pk
resume.employee.pk

Sources

• Django - Relationships
• Django - Making queries
• Entities for different types of relationships