database - Django: How do I model a tree of heterogeneous data types?

Question

I need to store a tree data structure in my database, for which I plan on using django-treebeard or possibly django-mptt. My source of confusion is that each node could be one of three different possible types: root nodes will always be a type A entity, leaf nodes a type C entity, and anything in between will be a type B entity. I would like to know the best way to model this situation.

update: I first tried model inheritance, and I think that this could be the best way to go. Unfortunately django-treebeard's public API isn't really designed to handle this. I ended up getting it to work with GenericForeignKey. Thank you very much for the answers.

Answer 1

Your three types are probably easiest handled as FK associations with the fundamental tree.

The tree can be homogenous -- class MyNode is a direct subclass of treebeard.Node. Your node can have a flag (Root, Middle, Leaf) and FK's for A or B or C. This allows you some SQL-like flexibility in querying MyNode instance.

This allows your tree to grow. A node can start as a type C (leaf) and then morph to a type B (intermediate). You change the status, and change the FK's.

The alternative is a bit more complex.

class MyA( treebeard.Node ):
    pass

class MyB( treebeard.Node ):
    pass

class MyC( treebeard.Node ):
    pass

In this case, you can't "morph" a node. When a node starts as a MyC, and gets children, you have to remove the original MyC instance, and replace it with a MyB version that has a new node as a child. This isn't impossible, but it can be painful.