boards/models/banner.py

from django.db import models


class Banner(models.Model):
    title = models.TextField()
    text = models.TextField(blank=True, null=True)
    post = models.ForeignKey('Post', on_delete=models.CASCADE)

    def __str__(self):
        return self.title

    def get_text(self) -> str:
        return self.text or self.post.get_text()