[Databases support] Implement declarative base Model class

[rszamszur]

• I have searched the issues of this repo and believe that this is not a duplicate.

Feature Request

In order to add support for databases with SQLAlchemy, a Base model class needs to be implemented from which concrete models will be inheriting.

Resources:

• https://fastapi.tiangolo.com/advanced/async-sql-databases/
• https://fastapi.tiangolo.com/tutorial/sql-databases/
• https://docs.sqlalchemy.org/en/20/orm/mapping_styles.html
• https://pydantic-docs.helpmanual.io/usage/models/#orm-mode-aka-arbitrary-class-instances

To think through:

• How to implement SQLAlchemy session? A separate class that will be initialized on the FastAPI startup event handler and passed to the controller via FastAPI Depends() (or import). Or implemented into base model class.
• How long keep an open session? Perpetual per worker? Or per request with context manager for db connection?
• Implement dynamically generation of pydantic model classes for FastAPI endpoints (These Pydantic models define more or less a "schema" for endpoints to have common attributes while reading data.)? Or hard coded in separate files?

This still needs some more research and clarification.