Arcanus

Arcanus is a small library that binds Pydantic schemas to your datasource. The idea is to let you work with one set of typed, validated objects that are backed by your real backend records — so the templates, factories, and converters that usually sit between validation and persistence mostly fall away. If that boilerplate has bothered you too, this is the approach Arcanus takes to it.

It's a different take from SQLModel, which fuses the Pydantic model and the ORM table into one class. Arcanus instead keeps them separate and binds a schema to your existing ORM model with bless(), so your SQLAlchemy models stay untouched and your validation schema stays your own.

⚠️ Work in progress. Arcanus is at an early, minimum-viable stage — expect bugs and breaking changes. SQLAlchemy is currently the only supported backend.

Features

• 🔄 Unified objects — transmuters are backed by ORM objects; changes sync both ways, no manual conversion.
• 🛡️ Type safety — full Pydantic validation, plus typed column references like Author["name"].
• 🔗 Relationships — one-to-one, one-to-many, many-to-many, and keyed maps that respect lazy loading.
• ⚡ Async — native async/await via AsyncSession.
• 🎯 Pluggable backends — NoOpMateria for tests, SqlalchemyMateria for databases.
• 📦 Partial models — built-in Create / Update shapes for API boundaries.

Installation

pip install "arcanus[sqlalchemy]"

The base pip install arcanus ships the in-memory NoOpMateria (no backend needed) — ideal for tests and prototyping.

Quickstart

from arcanus.base import BaseTransmuter, Identity
from arcanus.association import (
    Relation,
    RelationCollection,
    Relationship,
    Relationships,
)
from arcanus.materia.sqlalchemy import SqlalchemyMateria, Session
from pydantic import Field
from sqlalchemy import create_engine, select
from typing import Annotated, Optional

materia = SqlalchemyMateria()


@materia.bless(AuthorModel)  # bind to your existing SQLAlchemy model
class Author(BaseTransmuter):
    id: Annotated[Optional[int], Identity] = Field(default=None, frozen=True)
    name: str
    books: RelationCollection["Book"] = Relationships()


@materia.bless(BookModel)
class Book(BaseTransmuter):
    id: Annotated[Optional[int], Identity] = Field(default=None, frozen=True)
    title: str
    author: Relation[Author] = Relationship()


with Session(create_engine("sqlite://")) as session:
    author = Author(name="Isaac Asimov")
    session.add(Book(title="Foundation", author=Relation(author)))
    session.commit()

    # Plain SQLAlchemy reads — only the results come back as transmuters
    found = session.execute(
        select(Author).filter_by(name="Isaac Asimov")
    ).scalar_one()  # a transmuter, not a raw ORM row
    for book in found.books:  # loads on access
        print(book.title, book.author.value is found)  # identity preserved

See the Quickstart guide for the full walkthrough including the ORM model definitions and async usage.

Documentation

Full documentation lives at kalynnka.github.io/arcanus:

• Why Arcanus — the problem it solves.
• Quickstart — define a transmuter and run CRUD.
• The Materia System — binding and design philosophy.
• API Reference — the full public API.

Contributing

See CONTRIBUTING.md for branch naming, conventional commits, and the release flow.

License

MIT