Libraries for common business logic - best practice?

Hi,

I am working on my first large rails app and had a quick question of best practice.

It is essentially an ERP type application, tracking customer accounts, orders, and product inventory.

I am finding I am learning towards creating method libraries containing certain areas of business logic that refer to multiple models to achieve their result. They don't necessarily create or modify existing resources, more make a decision based on some business rules.

Pricing for example. Given the complexity of product pricing models with customer specific pricing I was thinking of having some form of pricing engine that given a customer and a product returned the price chargeable.

I am also looking at doing similar things with despatch methods and so forth.

Essentially encapsulating a number of business rules which do not necessarily belong in the models.

Given the above I had considered creating modules within the lib directory and requiring them in the controllers as relevant.

Does anyone know of a better ruby or rails specific way of modeling this functionality?

Thanks, Andrew.