Building an ORM from Scratch in Python

Why Build an ORM

Using an ORM is easy. Understanding one is different. Building a minimal ORM from scratch helped me see how models become tables, how fields map to columns, and how Python internals support clean developer APIs.

The project focused on model fields, descriptors, metaclasses, SQLite integration, and simple query logic.

Descriptors and Fields

Descriptors let a class control how attribute access works. That makes them a natural fit for model fields.

class Field:
    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, instance, owner):
        return instance.__dict__.get(self.name)

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value

Metaclasses and Model Mapping

A metaclass can inspect model definitions when the class is created. That is where field collection and table mapping can happen.

Takeaway

Building the small version makes the large version less magical. Django ORM and SQLAlchemy are deep systems, but the core idea starts with mapping Python objects to database rows in a predictable way.