Dynamically populate constraints in Field.contribute_to_class

covracer · April 24, 2026, 1:50pm

Here’s what I found when digging in a while back:

I think a neat example is bounds checking for the battleship board game where each new game the players choose from predefined geometries like 10x5, 16x8, or 20x10. (Or maybe or tic tac toe, connect four, pen the pig, chess puzzles, etc. Square len(rows) == len(columns) will be simpler to model and check than non-square len(rows) != len(columns). A fuller example below:

github.com/biobuddies/wellplated

wellplated%2Ffields.py

main

"""
Models with consistent checks at every level

1. Database CheckConstraints
2. Python validators
3. HTML5 constraint validation attributes
   https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation
"""

from functools import partial
from typing import Any, Self, cast

from django.db.models import CharField, CheckConstraint, Model, PositiveSmallIntegerField, Q, Value
from django.db.models.functions import Cast, Left, Length, Substr
from django.db.models.options import Options
from django.forms import ChoiceField, Field

CharField.register_lookup(Length)

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wellplated/wellplated/models.py at main · biobuddies/wellplated · GitHub

github.com/biobuddies/wellplated

wellplated%2Fmodels.sql

main

CREATE TABLE IF NOT EXISTS "wellplated_format" (
    "id" integer NOT NULL PRIMARY KEY AUTOINCREMENT,
    "bottom_row" varchar(1) NOT NULL,
    "right_column" smallint unsigned NOT NULL CHECK ("right_column" >= 0),
    "prefix" varchar(11) NOT NULL UNIQUE,
    "bottom_right_prefix" varchar(14) GENERATED ALWAYS AS (
        (
            COALESCE("bottom_row", '')
            || COALESCE(
                (
                    COALESCE(LPAD(CAST("right_column" AS text), 2, '0'), '')
                    || COALESCE("prefix", '')
                ),
                ''
            )
        )
    ) STORED UNIQUE,
    "created_at" datetime NOT NULL,
    "purpose" text NOT NULL UNIQUE,
    CONSTRAINT "len(wellplated_format.bottom_row) == 1" CHECK (LENGTH("bottom_row") = 1),

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You may find people advocating that new and improved Fields begin and grow as third party installables, without burdening the Django Foundation with maintenance, and with the option to someday/maybe merge upstream.

I think many Django projects could benefit from more comprehensive SQL constraints. To scope creep a little: what about harmonized checks at every level? HTML attributes/JavaScript for the frontends like the admin interface, Python code like full clean on save, and SQL constraints as the backstop?

Topic		Replies	Views
I’m writing a custom model field and would like it to come with a database CHECK Using the ORM	2	112	March 9, 2025
CharField and Textfield default constraints? ORM	6	1882	November 29, 2023
DRY Meta.constraints Using the ORM	3	761	August 16, 2022
Fields excluded from model constraint validation Using the ORM	7	744	October 16, 2025
upgrading from old Django version 2.2. add PositiveIntegerField check constraint Django Internals	1	82	October 10, 2024

Dynamically populate constraints in Field.contribute_to_class

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