ForeignKey and on_delete in Django

from django.db import models

class Task(models.Model):
    project = models.ForeignKey(
        'Project',
        on_delete=models.CASCADE,  # delete the task when the project is deleted
    )

# Delete tasks when the project is deleted
project = models.ForeignKey(Project, on_delete=models.CASCADE)

# Prevent project deletion while tasks exist
project = models.ForeignKey(Project, on_delete=models.PROTECT)

# Task without a project
project = models.ForeignKey(
    Project,
    on_delete=models.SET_NULL,
    null=True,
    blank=True,
)

class Task(models.Model):
    project = models.ForeignKey(
        Project,
        on_delete=models.CASCADE,
        related_name='tasks',  # project.tasks.all()
    )

project = Project.objects.get(pk=1)
tasks = project.tasks.all()        # via related_name
tasks = project.task_set.all()     # default (without related_name)