belongs-to relationship question

High level question, with a "belongs-to" relationship .... you usually put the primary key of the parent object into the table of the child. I would like to know if there is a way to reference the parent object, via something else than the primary key. Here is a simple example:

model: Class Property "belongs-to" class Neighborhood Class Neighborhood "has-many" class Properties

SQL DB: Properties Table contains a field called AREA. Neighborhood Table contains a field called AREA_ID

Property.AREA == Neighborhood.AREA_ID // these two data points match

Can I reference the Neighborhood Object from the Property Object, without adding the primary key from the Neighborhood table to the property table? Ideally, I would like to write code like: <% %>, as I would be able to if I added the primary Neighborhood id key to the Property table.

Thanks for any insight!


Is Neighborhood.area unique for a given neighborhood, i.e. not two neighborhoods have the same area.

If so then it sounds like area should be the primary key for neighborhoods. If you have to name it area then you need to do something like (this is off the top of my head and untested):

1) Make it the actual primary key, if you are using migrations use the ;id => false option on create table something like:

   CreateNeighborhoods << ActiveRecord::Migration          def self.up                create_table :neighborhoods, :id => false do |t|                     t.add_column :AREA_ID, :primary                     ...

Although I'm guessing that this is probably a legacy DB otherwise why are you fighting ActiveRecord?

Note that this assumes that it's an autoincrement integer, if not you've got some more working against conventions to figure out on your own.

2) override the conventions in the Neighborhood model

    class Neighborhood << ActiveRecord::Base

        primary_key = "AREA_ID"         has_many :properties

   class Property        belongs_to :neighborhood, :foreign_key => "AREA_ID"

This should be:

          set_primary_key "AREA_ID"

I DID say that it was off the top of my head.