Yesterday, I was trying to set a unique constraint for comments in Buttondown to prevent accidental double-commenting, and I ran into a problem that I hadn't seen before:

index row size 2816 exceeds btree version 4 maximum 2704 for index "emails_comment_email_id_subscriber_id_text_0542cca9_uniq"
DETAIL:  Index row references tuple (165,7) in relation "emails_comment".
HINT:  Values larger than 1/3 of a buffer page cannot be indexed.
Consider a function index of an MD5 hash of the value, or use full text indexing.

Simple enough: indexing a very long string is going to be prohibitively bad. It was immediately clear that the right path forward was to index the MD5 hash of the text rather than the text itself, but the literature on how to do so within the ORM was somewhat lacking:

• A decades-old tracking ticket had nothing useful
• StackOverflow either recommended dropping down to raw SQL or didn't think it was possible for uniqueness constraints

However, the solution is actually quite easy! Since Django 4.0, you can use expression-based uniqueness constraints, and Django even offers a handy MD5 function right out of the box. All I had to do was this:

from django.db.models import UniqueConstraint
from django.db.models.functions import MD5

class Comment(models.Model):
    text = models.TextField()
    email = models.EmailField()

    class Meta:
        constraints = [
            models.UniqueConstraint(MD5("text"), "email", name="unique_text_email_idx")
        ]

And that's it!