Custom validation in Django admin

Usually you just want to define a clean() method on the model itself.

https://docs.djangoproject.com/en/2.1/ref/models/instances/#validating-objects

from django.core.exceptions import ValidationError

class Lecture(models.Model):
    topic = models.CharField(max_length=100)
    speaker = models.CharField(max_length=100)
    start_date = models.DateField()
    end_date = models.DateField()

    def clean(self):
        if self.start_date > self.end_date::
            raise ValidationError("Dates are incorrect")

Something like that will work in the django admin without any need to create a form class.