tiangolo.fastapi/docs/tutorial/extra-models.md

Continuing with the previous example, it will be common to have more than one related model.

This is especially the case for user models, because:

• The input model needs to be able to have a password
• The output model should do not have a password
• The database model would probably need to have a hashed password

!!! danger Never store user's plaintext passwords. Always store a secure hash that you can then verify.

Multiple models

Here's a general idea of how the models could look like with their password fields and the places where they are used:

{!./tutorial/src/extra-models/tutorial001.py!}

!!! warning The supporting additional functions are just to demo a possible flow of the data, but they of course are not providing any real security.

Reduce duplication

Reducing code duplication is one of the core ideas in FastAPI.

As code duplication increments the chances of bugs, security issues, code desynchronization issues (when you update in one place but not in the others), etc.

And these models are all sharing a lot of the data and duplicating attribute names and types.

We could do better.

We can declare a Userbase model that serves as a base for our other models. And then we can make subclasses of that model that inherit its attributes (type declarations, validation, etc).

All the data conversion, validation, documentation, etc. will still work as normally.

That way, we can declare just the differences between the models (with plaintext password, with hashed_password and without password):

{!./tutorial/src/extra-models/tutorial002.py!}

Recap

Use multiple Pydantic models and inherit freely for each case. You don't need to have a single data model per entity if that entity must be able to have different "states". As the case with the user "entity" with a state including password, password_hash and no password.