Question

I’m confused on how I should make this scope or method. I have the following associations:

Models

class User
  has_many :prices
  has_many :products, :through => :prices
  has_many :subscriptions, :foreign_key => :subscriber_id
end

class Product
  has_many :prices
  has_many :users, :through => :prices
end

class Price
  # Table columns => :product_id, :cost, :user_id
  belongs_to :user
  belongs_to :product
  belongs_to :store
  has_many :subscriptions, :as => :subscribable
end

class Subscription
  # Table columns => :product_id, :cost, :subscriber_id, :subscribable_id
  # :subscribable_type
  belongs_to :subscriber, :class_name => "User"
  belongs_to :subscribable, :polymorphic => true
  validates_uniqueness_of :subscribable_id, :scope => 
                        [ :subscriber_id, :subscribable_type]
end

So the method should be something like:

class Price

def self.lower_price
  if self.product_id == self.subscription.product_id
     if self.cost < self.subscription.cost
     end
  end
end

end

What this method is suppose to do is show only lower prices of UserProducts that belong to the same Product as the Subscription, while comparing itself to the subscriptions price field to see if its lower.

Am I doing this right? What needs to be fixed?

---

EDIT

class Price < ActiveRecord::Base
  scope :for_product, lambda { |product_id| where( :product_id => product_id) }
  scope :cheaper, lambda { |cost| where(["prices.cost < :cost", { :cost => cost } ] ) }
end

class Subscription < ActiveRecord::Base

  def cheaper_prices
    Price.for_product(product_id).cheaper(cost)
  end
end

PrivatePagesController:

def watch
 @prices = Price.cheaper_prices.paginate(:page => params[:page], :per_page => 20).order('purchase_date DESC')
end

This gives me the error:

NoMethodError in PrivatePagesController#watch

undefined method `cheaper_prices' for #<Class:0x6f99210>

Answer 1

I figured that you are making a web site where users enter prices they found and subscribe to find cheaper prices. I’d rename the UserProduct entity to Price.

It’s ambiguous whether subscribers subscribe to a product or to a price. If you clear that up, it may simplify away the polymorphic association. Let’s say that they subscribe to a product with a given price. Then you want the following:

class Price
  # Table columns => :product_id, :price, :user_id
  belongs_to :finder, :class_name => "User"
  belongs_to :product
  belongs_to :store

  scope for_product, lambda { |product_id| where(:product_id => product_id)
  scope cheaper, lambda { |price| where([ "prices.price < :price", {:price => price} ] }
end

class Subscription
  # Table columns => :product_id, :price, :subscriber_id
  belongs_to :subscriber, :class_name => "User"
  belongs_to :product
  validates_uniqueness_of :subscribable_id, :scope => 
                        [ :subscriber_id, :subscribable_type]

  def cheaper
    Price.for_product(product_id).cheaper(price)
  end
end