Alternative to a union with ActiveRecord - sql

Question

I think I want to do a union in Rails, but according to this post rails union hack, how to pull two different queries together unions aren't natively supported in Rails. I'm wondering if there is a better way to approach this problem.
I have table of items, each item has many prices, but I only want to join one price to each item.
To determine the proper price for an item I have two additional foreign keys in the price model: category_id and discount_id. Each could independently declare a price for an item.
Ex.
Item + Category = Price1 and Item + Discount = Price 2
If discount_id matches a passed id I want to exclude the price results FOR THAT ITEM ONLY that match Item + Category. Also I'm trying not to loose lazy loading.
Hopefully the problem is clear! If not I'll try to clarify more, thanks in advance.

Answer 1

Your models would start off looking something like this:
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
One way of doing this is to add a class method to Price that encapsulates this logic:
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
This is effectively the same as this query:
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)
You can add these helper methods on your Item model for simplicity:
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
Now you can easily get each individual 'type' of price for a single item (will be nil for prices that aren't available):
item.category_price
item.discount_price
item.used_price