What is the best way to model user defined hierarchical relationships in a database?

Question

Essentially, I want the user to be able to define a hierarchical model, but then I need to allow the user to store data within their defined model. Does this make sense? So the

Answer 1

This is an extremely broad question, but this may point you in the right direction. Note that you're only going to be able to store the relationship rules in the database. Enforcing them will be up to your client code. Try this on for size..

unit:
    unit id,
    name,

unit relationship:
    unit id,
    foreign unit id

You can then use your unit relationship table in the following way..

unit id relates to the unit it's describing. foreign unit id should be nullable.

A unit with no relationship records can only exist at the root of the heirarchy. A unit with a null foreign unit id can have any other unit as its parent. Otherwise, a unit must have another unit as its parent, and it's type must be one of those defined in its relationship records.

As for storing the instances themselves, that should be straightforward..

instance:
    instance id,
    unit id,
    parent instance_id

I'm sure there would be other fields you'd need (name, for instance), but I assume you get the drift.