get_issues

models.py

class Sprint(UuidModel, TimeStampedModel):
    title = models.CharField(max_length=100, null=True, blank=True)
    project = models.ForeignKey(
        Project,
        on_delete=models.CASCADE,
        related_name='sprints',
    )

    def get_issues(self):
        return self.issue_set.all()


class Issue(TimeStampedModel, UuidModel):
    number = models.PositiveIntegerField()
    title = models.CharField(max_length=255)
    sprint = models.ForeignKey(
        Sprint,
        on_delete=models.CASCADE,
    )


class Project(TimeStampedModel, Active):
    title = models.CharField(max_length=255, unique=True)

    def get_sprints(self):
        return self.sprints.all()

    def get_issues(self):
        return [sprint.get_issues() for sprint in self.get_sprints()]