Inheritance for SQLAlchemy Models

In software engineering one of the key principles of object oriented software is the concept of inheritance. It can be used to increase code re-use which reduces the volume of tests and speeds up development. You can use inheritance in SQLAlchemy as described here. However, this inheritance is mainly used to describe relationships between tables and not for the purpose of re-using certain pieces of models defined elsewhere.

The openapi specification allows for inheritance using the allOf statement. This means that you could, for example, define a schema for id properties once and re-use that schema for any number of objects where you can customise things like the description that may differ object by object. You can also use allOf to combine objects, which is a powerful way of reducing duplication. You could, for example, define a base object with an id and name property that you then use repeatedly for other objects so that you don’t have to keep giving objects an id and a name.

If this feature could be brought to SQLAlchemy models, you would have a much shorter models.py files which is easier to maintain and understand. The plan for the openapi-SQLAlchemy package is to do just that. The first step has been completed with the addition of support for allOf for column definitions. If you aren’t familiar with the package, the Reducing API Code┬áDuplication article describes the aims of the package.

To start using the column inheritance feature, read the documentation for the feature which describes it in detail and gives and example specification that makes use of it.