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Buy One Get One Promotion with Spree

Implementing a "Buy One, Get One Free" promotion in Spree requires implementation of a custom promotion action and appropriate use of existing promotion rules. This article implements the promotion by automatically adding and removing immutable "get one" line items whose price is zero and whose quantity always mirrors its paid "buy one" counterpart. Although written and tested with Spree's 1-3-stable branch, the core logic of this tutorial will work with any version of Spree.

Promotion Eligibility

Begin by creating a new promotion using a meaningful name. Set the "event name" field to be "Order contents changed" so the promotion's actions are updated as the order is updated. Save this new promotion, so we can then configure Rules and Actions. In the Rules section, select the "Product(s)" rule and click Add. Now choose the products you'd like to be eligible for your promotion. If you'd like to include broader sets such as entire taxonomies (and have implemented the custom promotion rules to do so), feel free to use them. When we implement the promotion action, you'll be able to make things work.

You should now have a product rule that selects some subset of products eligible for your promotion.

Adding a Custom Promotion Action

We'll now add a custom promotion action that will do the work of creating the free line items for each eligible paid line item. Again, this implementation is specifically for the 1-3-stable branch, but the public interface for promotion actions has (amazingly) remained stable, and is supported from 0-7-0-stable all the way through 2-0-stable.

First, we begin by doing the basic wiring for creating a new promotion action. As the guide instructs, create a new promotion class in a file.

# app/models/spree/promotion/buy_one_get_one.rb
class BuyOneGetOne < Spree::PromotionAction
  def perform(options={})
    # TODO
  end
end

Then register this new class with Spree using an initializer.

# config/initializers/spree.rb

Rails.application.config.spree.promotions.actions << BuyOneGetOne

Then update your locales to provide translations for the promotion action's name and description.

# config/locales/en.yml

en:
  spree:
    promotion_action_types:
      buy_one_get_one:
        name: Buy One Get One
        description: Adds free line items of matching quantity of eligible paid line items.

And although the guide doesn't instruct you to, it seems required that you add an empty partial to be rendered in the Spree admin when you select the rule.

# app/views/spree/admin/promotions/actions/_buy_one_get_one.html.erb

# Empty File. Spree automatically renders the name and description you provide,
# but you could expand here if you'd like.

Pre-flight check

Before moving any farther, it's best to make sure the new promotion action has been wired up correctly. Restart your development server so the initializer registers your new promotion action and refresh your browser. You should now see a "Buy One Get One" promotion action.

Buy One Get One Logic

Now that we've got the promotion action wired, we're ready to implement the logic needed to create the new line items. Begin by collecting the order from the options.

# app/models/spree/promotion/buy_one_get_one.rb
class BuyOneGetOne < Spree::PromotionAction
  def perform(options={})
    return unless order = options[:order]
  end
end

Next we need to determine which line items in the order are eligible for a corresponding free line item. Because line items are for variants of products, we must collect the variant ids from the product rule we setup. If you've used something other the default Spree "Product(s)" rule, just make sure you end up with equivalent output.

# app/models/spree/promotion/buy_one_get_one.rb
class BuyOneGetOne < Spree::PromotionAction
  def perform(options={})
    return unless order = options[:order]

    eligible_variant_ids = get_eligible_variant_ids_from_promo_rule
    return unless eligible_variant_ids.present?
  end

  def get_eligible_variant_ids_from_promo_rule
    product_rule = promotion.promotion_rules.detect do |rule|
      # If not using the Product rule, update this line
      rule.is_a? Spree::Promotion::Rules::Product
    end 
    return unless product_rule.present?

    eligible_products = product_rule.products
    return unless eligible_products.present?

    eligible_products.collect { |p| p.variant_ids }.flatten.uniq
  end
end

Now that we've got the eligible variant ids from the promotion's rule, we'll identify the line items which have those variants.

# app/models/spree/promotion/buy_one_get_one.rb
class BuyOneGetOne < Spree::PromotionAction
  def perform(options={})
    return unless order = options[:order]

    eligible_variant_ids = get_eligible_variant_ids_from_promo_rule
    return unless eligible_variant_ids.present?

    order.line_items.where(variant_id: eligible_variant_ids).each do |li|
      #TODO
    end
  end
  ....
end

We're now ready to process eligible line items. There are several cases we'll need to implement to have a working promotion:

  • When we find an eligible, paid line item:

    • If an existing corresponding free line item exists, update quantity to match the paid line item.

    • Else, create corresponding free line item with appropriate quantity.

  • When we find a Buy One Get One promotion line item:

    • If the corresponding paid line item still exists, do nothing.

    • Else, destroy the free line item.

These cases handle the creation, updating, and removal of promotional line items. Let's translate these cases into a skeleton of code which can then be implemented incrementally.

# app/models/spree/promotion/buy_one_get_one.rb
class BuyOneGetOne < Spree::PromotionAction
  def perform(options={})
    return unless order = options[:order]

    eligible_variant_ids = get_eligible_variant_ids_from_promo_rule
    return unless eligible_variant_ids.present?

    order.line_items.where(variant_id: eligible_variant_ids).each do |li|
      if li.price != 0
        # It's an eligible variant and it's price is not zero, so we
        # found a "buy one" line item.


        matching_get_one_line_item = find_matching_get_one_line_item(li)

        # Create or update matching promo line item.
        if matching_get_one_line_item
          matching_get_one_line_item.update_attribute(:quantity, li.quantity)
        else
          create_matching_get_one_line_item(li)
        end 

      else
        # It's an eligible variant and it's price is zero, so we
        # found a "get one" line item.

        # Verify "buy one" line item still exists, else destroy
        # the "get one" line item
        li.destroy unless find_matching_buy_one_line_item(li)
    end
  end

  def find_matching_buy_one_line_item(li)
  end
  def create_matching_get_one_line_item(li)
  end
  def find_matching_get_one_line_item(li)
  end
  ....
end

This well named and commented code reads nicely and clearly covers the create, update, and destroy cases we need to be concerned with. Now let's implement the helper methods.

  ....
  def find_matching_buy_one_line_item(get_one_line_item)
    get_one_line_item.order.line_items.detect do |li|
      li.variant_id == get_one_line_item.variant_id and li.price != 0
    end 
  end

  def create_matching_get_one_line_item(buy_one_line_item)
    # You may need to update this with other custom attributes you've added.
    new_line_item = buy_one_line_item.order.line_items.build
    new_line_item.variant = buy_one_line_item.variant
    new_line_item.currency = buy_one_line_item.currency
    new_line_item.price = 0
    new_line_item.quantity = buy_one_line_item.quantity
    new_line_item.save
  end

  def find_matching_get_one_line_item(buy_one_line_item)
    buy_one_line_item.order.line_items.detect do |li|
      li.variant_id == buy_one_line_item.variant_id and li.price != 0
    end 
  end
  ....
end

We've now completed most of the implementation for the promotion action. Every time the order is updated, all line items are scanned for eligibility and the appropriate create/update/destroy actions are taken. This however, isn't the end of our implementation. Unlike other items in our cart, we need to prevent users from changing the quantity of "get one" line items or removing them from the cart. We also need some way of indicating that these zero price line items are complements of the Buy One Get One promotion.

Locked Line Items with Additional Text

While the concept of locked or immutable line items might rightly deserve its own, separate Spree plugin, we'll roll a quick one here to complete the implementation of our promotion. We'll need to add a few attributes to the spree_line_items database table, and tweak our implementation of create_matching_get_one_line_item.

# db/migration/add_immutable_and_additional_text_to_spree_line_items.rb
class AddImmutableAndAdditionalTextToSpreeLineItems < ActiveRecord::Migration
  def change
    add_column :spree_line_items, :immutable, :boolean
    add_column :spree_line_items, :additional_text, :string
  end
end

# app/models/spree/promotion/buy_one_get_one.rb
  def create_matching_get_one_line_item(buy_one_line_item)
    # You may need to update this with other custom attributes you've added.
    new_line_item = buy_one_line_item.order.line_items.build
    new_line_item.variant = buy_one_line_item.variant
    new_line_item.currency = buy_one_line_item.currency
    new_line_item.price = 0
    new_line_item.quantity = buy_one_line_item.quantity

    new_line_item.immutable = true
    new_line_item.additional_text = "Buy One Get One"

    new_line_item.save
  end

Now that we know which line items aren't meant to be edited by users, we can update our UI to not render the options to remove immutable line items or update their quantity. We can also display the additional text in the line line item when it's available.

Missing from this implementation is a way to secure the immutable line items from manipulation of the immutable line items POSTed parameters. While this might be required in other cases using immutable line items, because our promotion action sets the "get one" line items quantity with every order update, we don't need to worry about this issue in this case.

Test Driving

At this point, you can begin manual testing of your implementation, but of course automated testing is best. Ideally, we would have TDDed this against a failing integration test, but the testing infrastructural setup required to do this is beyond the scope of the article. What's worth sharing though, is the syntax of the assertions I've developed to inspect line items, so that you can implement something similar for your specific needs. Here's a snippet from an integration test to give you a sense of the DSL we've built up.

# spec/requests/buy_one_get_one_spec.rb

context "add eligible item to cart" do
  before do
    # setup cart
    visit_product_and_add_to_cart eligible_product
    visit "/cart"
  end   
      
  it "should add an identical, immutable, free item" do
    assert_line_item_count(2)
    assert_line_item_present(eligible_product.name, unit_price: undershirt_amount, unit_price: eligible_product.price, immutable: false)
    assert_line_item_present(eligible_product.name + " - (Buy One Get One)", unit_price: 0.00, immutable: true)
    assert_order_total(eligible_product.price)
  end
end

Of course you'd want to test all the cases we've implemented, but what's worth focusing on is the ability to assert specific attributes across many different line items. This is an extremely reusable tool to have in your testing suite. Good luck implementing!

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