Question

I have a model Thing which can be subclassed by several types of Thing like PointyThing and TastyThing. I have a second model, Instance that is related one-to-many to Things (one Instance may be of the type of a single Thing but there will be many Instances of a given Thing). Instances are then related to a Player (each instance has one Player but a player has many Instances) with a backref so that a Player can call it’s .inventory property to see what it owns.

All well and good, but I also have a model Place. I would like for Places to own Instances too in the same way that a Player owns an instance.

Would it be best to create an Owner model that is linked against with the Instance model and then subclassed to get Players and Places or some hereto unknown method within SQLAlchemy that I don’t yet know of?

Answer 1

I think the best solution you are asking for depends on many factors.

Technically, if I look at your example in isolation, this solution looks like a pretty descent hack which avoids creating another relationship table. And if you will never query with polymorphic support, this could fly just fine. However it would still be a hack. Imagine that later you extend your Player model with few more sub-classes, and you might start using polymorphic queries, and you will always have to ask yourself "how will it impact my hack"?. And even if everything still would work fine (right now I cannot come up with example which would break your logic), you would still need to be careful.
But lets look what is the benefit of this hack? We save on one relationship table, but in fact you introduce another table for your Owner model (I assume Concrete Table Inheritance), so what is the gain really?

On the other hand, I am wondering if your Instance table should not in fact be a ternary relationship? I assume that each instance of the Thing is stored in some Place and might belong to a Player, so it might just be one table that looks like:

Instance[
    ID primary_key, 
    Thing_ID (FK) NOT NULL, 
    Place_ID (FK) NOT NULL, 
    Person_ID (FK) NULL
]

Note that Person_ID is nullable, as I assume the instance of the Thing might not belong to anyone until assigned. But it could be that it is always NOT NULL in your case.

Hope this helps. Would be good to learn which way you decide to go and why.