You can mimic the belongs_to behaviour to do what you want. All that belongs_to does is create a method that returns an object that match the criteria. The clever bit is the way it uses the Rails conventions to work out what needs to be returned with the minimum of effort.
So if you have a Child class that contains:
The system will work out from normal Rails conventions that there will be two tables (children and parents) and that the children table will have a parent_id field. It will then be able to automatically generate the SQL code that will return the Parent object that is associated to a particular Child via its parent_id via something like:
where 3 in the entry in parent_id for the current child.
SELECT * FROM parents WHERE id = 3
You could replicate this behaviour with a method like this in the Child model:
Therefore, for your instance, what you need to do is mimic the parent method created via with you're own custom parent method that alters the Parent.find criteria to suit your custom database.
Parent.find(:first, :conditions => ["id = ?", parent_id])
Perhaps something along the lines of:
You may also want to mimic "has_many :children" by creating a children method in your Parent class with something like:
Valve.find(:first, :conditions => ["NPSMinKey = ? AND NPSMaxKey =?", something, something_else])
Which will return an array of Child objects whose parent_id match the current Parent's id.
Child.find(:all, :conditions => ["parent_id = ?, id])