Non-generic relations

[jacobjove]

Add abstract models to enable users to implement non-generic relations between entities, attributes, and values.

Detailed Description

I would like to define my own relations between entities (of known types) and attributes and values, rather than using Django's contenttypes framework and generic relations.

Context

If this library offers abstract models (in addition to, and as an alternative to, the concrete models that define generic relations), users could benefit from this library while (arguably) enjoying better customizability and performance.

Possible Implementation

I'm working on a draft PR.