Question

I’m setting up an app for posting items wanted/available/bartering. I’ve got two main classes, Post and Item.

Each post contains an offered_item, a wanted_item, or both. Here are my current definitions:

class Post < ActiveRecord::Base
  has_one :wanted_item, :class_name => 'Item', :dependent => :destroy
  has_one :offered_item, :class_name => 'Item', :dependent => :destroy
  has_one :location, :dependent => :destroy
end

and

class Item < ActiveRecord::Base
  belongs_to :post

  attr_accessible :title, :description
end

On to my question: How do I structure the foreign keys in the Items table such that I can tell which post (and whether it’s a wanted_item or offered_item)? Is this done in the migrations files or in the models?

As it currently sits, when I try a query like:

Post.find(:first).wanted_item 

I get the following:

SQLite3::SQLException: no such column: items.post_id: SELECT
“items”.* FROM “items” WHERE “items”.”post_id” = 1 LIMIT 1

Answer 1

You can set conditions on associations, like so:

has_one :wanted_item, :class_name => 'Item', :conditions => ['kind = ?', 'wanted']

and add a kind column to Item (don’t use “type”, it’s a reserved word).

EDIT: reading your post again, your Item table seems to be lacking the foreign key. In your migration file that creates your Item table, simply include t.references :post to have rails create a post_id foreign key column.