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copied!<p>Your models would start off looking something like this:</p> <pre><code>class Price < ActiveRecord::Base belongs_to :item belongs_to :category belongs_to :discount scope :category, where("prices.category_id IS NOT NULL") scope :discount, where("prices.discount_id IS NOT NULL") end class Item < ActiveRecord::Base has_many :prices end class Category < ActiveRecord::Base has_many :prices end class Discount < ActiveRecord::Base has_many :prices end </code></pre> <p>One way of doing this is to add a class method to <code>Price</code> that encapsulates this logic:</p> <pre><code>class Price < ActiveRecord::Base def self.used discount_items_sql = self.discount.select("prices.item_id").to_sql where("prices.discount_id IS NOT NULL OR prices.item_id NOT IN (#{discount_items_sql})") end end </code></pre> <p>This is effectively the same as this query:</p> <pre><code>SELECT * FROM prices WHERE prices.discount_id IS NOT NULL -- the discount_id is present on this record, OR prices.item_id NOT IN ( -- or no discount_id is present for this item SELECT item_id FROM prices WHERE discount_id IS NOT NULL) </code></pre> <p>You can add these helper methods on your <code>Item</code> model for simplicity:</p> <pre><code>class Item < ActiveRecord::Base def category_price prices.category.first end def discount_price prices.discount.first end def used_price prices.used.first end end </code></pre> <p>Now you can easily get each individual 'type' of price for a single item (will be <code>nil</code> for prices that aren't available):</p> <pre><code>item.category_price item.discount_price item.used_price </code></pre>

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