Add support for volume pricing based on bands

This allows discounts to be set on the actual quantities which end up in the specific range, with the rest calculated on the full price (or the respective discount set up for that band)

Author: sztupy

.circleci/config.yml

@@ -33,7 +33,7 @@ jobs:
   lint-code:
     executor:
       name: solidusio_extensions/sqlite
-      ruby_version: "3.0"
+      ruby_version: "3.1"
     steps:
       - solidusio_extensions/lint-code
 
@@ -43,15 +43,15 @@ workflows:
       - run-specs:
           name: &name "run-specs-solidus-<< matrix.solidus >>-ruby-<< matrix.ruby >>-db-<< matrix.db >>"
           matrix:
-            parameters: { solidus: ["main"], ruby: ["3.2"], db: ["postgres"] }
+            parameters: { solidus: ["main"], ruby: ["3.3"], db: ["postgres"] }
       - run-specs:
           name: *name
           matrix:
-            parameters: { solidus: ["current"], ruby: ["3.1"], db: ["mysql"] }
+            parameters: { solidus: ["current"], ruby: ["3.2"], db: ["mysql"] }
       - run-specs:
           name: *name
           matrix:
-            parameters: { solidus: ["older"], ruby: ["3.0"], db: ["sqlite"] }
+            parameters: { solidus: ["older"], ruby: ["3.1"], db: ["sqlite"] }
       - lint-code
 
   "Weekly run specs against main":
@@ -70,4 +70,4 @@ workflows:
       - run-specs:
           name: *name
           matrix:
-            parameters: { solidus: ["current"], ruby: ["3.1"], db: ["mysql"] }
+            parameters: { solidus: ["current"], ruby: ["3.1"], db: ["mysql"] }

README.md

@@ -15,14 +15,22 @@ Each VolumePrice contains the following values:
 1. **Variant:** Each VolumePrice is associated with a _Variant_, which is used to link products to
    particular prices.
 2. **Name:** The human readable representation of the quantity range (Ex. 10-100). (Optional)
-3. **Discount Type** The type of discount to apply.  **Price:** sets price to the amount specified.
-     * **Dollar:** subtracts specified amount from the Variant price.
-     * **Percent:** subtracts the specified amounts percentage from the Variant price.
+3. **Discount Type** The type of discount to apply.
+     * **Price:** sets price to the amount specified for all items
+     * **Dollar:** subtracts specified amount from the Variant price for all items
+     * **Percent:** subtracts the specified amounts percentage from the Variant price for all items
+     * **Banded Price:** sets price to the amount specified, but only for items within the range. For items outside the range it will consult that band to determine price
+     * **Banded Dollar:** subtracts specified amount from the Variant price, but only for items within the range. For items outside the range it will consult that band to determine price
+     * **Banded Percent:** subtracts the specified amounts percentage from the Variant price, but only for items within the range. For items outside the range it will consult that band to determine price
 4. **Range:** The quantity range for which the price is valid (See Below for Examples of Valid
    Ranges.)
 5. **Amount:** The price of the product if the line item quantity falls within the specified range.
 6. **Position:** Integer value for `acts_as_list` (Helps keep the volume prices in a defined order.)
 
+Note: when using percentage based or banded discounts then first the system will calculate a per-item price, and then get the total by multiplying the per-item price with the quantity. Due to rounding this can differ if the price would have been calculated on the total.
+
+Example: Percent discount is 10%, Original price is 9.99, Ordering 100 pieces. Per-item price is rounded down to $8.99 (from $8.991), so total will be $899.00 instead of $899.10
+
 ## Install
 
 The extension contains a rails generator that will add the necessary migrations and give you the
@@ -99,12 +107,68 @@ Cart Contents:
 | ------- | -------- | ----- | ----- |
 | Rails T-Shirt | 20 | 17.99 | 359.80 |
 
-## Additional Notes
+## Banded Examples
+
+Consider the following examples of volume prices:
+
+| Variant | Name | Type | Range | Amount | Position |
+| ------- | ---- | ---- | ----- | ------ | -------- |
+| Rails T-Shirt | 1-5 | Price | (1..5) | 19.99 | 1 |
+| Rails T-Shirt | 6-9 | Price | (6...10) | 18.99 | 2 |
+| Rails T-Shirt | 10-19 | Banded Percent | (10-19) | 50% | 3 |
+| Rails T-Shirt | 20 or more | Banded Percent | (20+) | 75% | 4 |
+
+### Example 1
+
+Cart Contents:
+
+| Product | Quantity | Price | Total |
+| ------- | -------- | ----- | ----- |
+| Rails T-Shirt | 1 | 19.99 | 19.99 |
+
+### Example 2
+
+Cart Contents:
+
+| Product | Quantity | Price | Total |
+| ------- | -------- | ----- | ----- |
+| Rails T-Shirt | 5 | 19.99 | 99.95 |
+
+### Example 3
+
+Cart Contents:
+
+| Product | Quantity | Price | Total |
+| ------- | -------- | ----- | ----- |
+| Rails T-Shirt | 6 | 18.99 | 113.94 |
+
+### Example 4
+
+Cart Contents:
+
+| Product | Quantity | Price | Total |
+| ------- | -------- | ----- | ----- |
+| Rails T-Shirt | 10 | 18.09 | 180.90 |
+
+Items #1-9 will be priced according to the `(5..9)` rule as it is unbanded. Their price will be $170.91
+
+Item #10 will be priced according to the `(10+)` rule, which describes a 50% reduction to the base price, which is $9.99 (rounded down)
+
+### Example 5
+
+Cart Contents:
+
+| Product | Quantity | Price | Total |
+| ------- | -------- | ----- | ----- |
+| Rails T-Shirt | 20 | 13.79 | 275.80 |
+
+Items #1-9 will be priced according to the `(5..9)` rule as it is unbanded. Their price will be $170.91
+
+Items #10-19 will be priced according to the `(10-19)` rule, which describes a 50% reduction to the base price. This equates to $9.99 per item (rounded down)
+
+Item #20 will be priced according to the `(20+)` rule, which describes a 75% reduction to the base price. This would be $4.99 (rounded down)
 
-* The volume price is applied based on the total quantity ordered for a particular variant. It does
-  not apply different prices for the portion of the quantity that falls within a particular range.
-  Only the one price is used (although this would be an interesting configurable option if someone
-  wanted to write a patch.)
+A per-item price of $13.79 is calculated (rounded down from $13.792875), then multiplied by the quantity getting $275.80. Note: this is $0.05 lower than what you would get if you total up the items separately, see notes above.
 
 ## License
 

app/models/solidus_volume_pricing/pricer.rb

@@ -10,17 +10,17 @@ def self.pricing_options_class
 
     def price_for(pricing_options)
       extract_options(pricing_options)
-      ::Spree::Money.new(computed_price)
+      ::Spree::Money.new(computed_price(pricing_options.quantity))
     end
 
     def earning_amount(pricing_options)
       extract_options(pricing_options)
-      ::Spree::Money.new(computed_earning)
+      ::Spree::Money.new(computed_earning(pricing_options.quantity))
     end
 
     def earning_percent(pricing_options)
       extract_options(pricing_options)
-      computed_earning_percent.round
+      computed_earning_percent(pricing_options.quantity).round
     end
 
     private
@@ -46,50 +46,48 @@ def volume_prices
       ::Spree::VolumePrice.for_variant(variant, user: user)
     end
 
-    def volume_price
+    def get_volume_price_for(quantity)
       volume_prices.detect do |volume_price|
         volume_price.include?(quantity)
       end
     end
 
-    def computed_price
-      case volume_price&.discount_type
-      when 'price'
-        volume_price.amount
-      when 'dollar'
-        variant.price - volume_price.amount
-      when 'percent'
-        variant.price * (1 - volume_price.amount)
+    def computed_price(quantity)
+      volume_price = get_volume_price_for(quantity)
+      band_price = case volume_price&.discount_type
+                   when /price/
+                     volume_price.amount
+                   when /dollar/
+                     variant.price - volume_price.amount
+                   when /percent/
+                     variant.price * (1 - volume_price.amount)
+                   else
+                     variant.price
+                   end
+
+      if volume_price&.discount_type&.starts_with?('banded_')
+        range_start = volume_price.begin
+        amount = quantity - range_start + 1
+        band_total = amount * band_price
+        if range_start > 1
+          band_total += computed_price(range_start - 1) * (range_start - 1)
+        end
+        band_total / quantity
       else
-        variant.price
+        band_price
       end
     end
 
-    def computed_earning
-      case volume_price&.discount_type
-      when 'price'
-        variant.price - volume_price.amount
-      when 'dollar'
-        volume_price.amount
-      when 'percent'
-        variant.price - (variant.price * (1 - volume_price.amount))
-      else
-        0
-      end
+    def computed_earning(quantity)
+      total_with_discount = computed_price(quantity) * quantity
+      total_without_discount = variant.price * quantity
+      (total_without_discount - total_with_discount) / quantity
     end
 
-    def computed_earning_percent
-      case volume_price&.discount_type
-      when 'price'
-        diff = variant.price - volume_price.amount
-        diff * 100 / variant.price
-      when 'dollar'
-        volume_price.amount * 100 / variant.price
-      when 'percent'
-        volume_price.amount * 100
-      else
-        0
-      end
+    def computed_earning_percent(quantity)
+      total_with_discount = computed_price(quantity) * quantity
+      total_without_discount = variant.price * quantity
+      100 - (total_with_discount * 100 / total_without_discount)
     end
   end
 end

app/models/spree/volume_price.rb

@@ -11,7 +11,7 @@ class VolumePrice < ApplicationRecord
     validates :discount_type,
       presence: true,
       inclusion: {
-        in: %w(price dollar percent)
+        in: %w(price dollar percent banded_price banded_dollar banded_percent)
       }
 
     validate :range_format
@@ -31,6 +31,7 @@ def self.for_variant(variant, user: nil)
     end
 
     delegate :include?, to: :range_from_string
+    delegate :begin, to: :range_from_string
 
     def display_range
       range.gsub(/\.+/, "-").gsub(/\(|\)/, '')

app/views/spree/admin/volume_prices/_volume_price_fields.html.erb

@@ -8,7 +8,10 @@
     <%= f.select :discount_type, [
         [t('spree.total_price'), 'price'],
         [t('spree.percent_discount'), 'percent'],
-        [t('spree.price_discount'), 'dollar']
+        [t('spree.price_discount'), 'dollar'],
+        [t('spree.banded_total_price'), 'banded_price'],
+        [t('spree.banded_percent_discount'), 'banded_percent'],
+        [t('spree.banded_price_discount'), 'banded_dollar']
       ], { include_blank: true }, class: 'custom-select' %>
   </td>
   <td>

config/locales/en.yml

@@ -11,10 +11,13 @@ en:
     new_volume_price_model: New Volume Price Model
     volume_price_model_edit: Edit Volume Price Model
     volume_pricing: Volume Pricing
-    price_discount: Price Discount
-    percent_discount: Percent Discount
     bulk_discount: Bulk Discount
-    total_price: Total price
+    price_discount: Price Discount (All items)
+    percent_discount: Percent Discount (All items)
+    total_price: Total price (All items)
+    banded_price_discount: Price Discount (Banded)
+    banded_percent_discount: Percent Discount (Banded)
+    banded_total_price: Total price (Banded)
     admin:
       tab:
         volume_price_models: Volume Price Models

spec/features/manage_volume_price_models_feature_spec.rb

@@ -13,7 +13,7 @@
     fill_in 'Name', with: 'Discount'
     within '#volume_prices' do
       fill_in 'volume_price_model_volume_prices_attributes_0_name', with: '5 pieces discount'
-      select 'Total price', from: 'volume_price_model_volume_prices_attributes_0_discount_type'
+      select 'Total price (All items)', from: 'volume_price_model_volume_prices_attributes_0_discount_type'
       fill_in 'volume_price_model_volume_prices_attributes_0_range', with: '1..5'
       fill_in 'volume_price_model_volume_prices_attributes_0_amount', with: '1'
     end

spec/features/manage_volume_prices_feature_spec.rb

@@ -13,7 +13,7 @@
     expect(page).to have_content('Volume Prices')
 
     fill_in 'variant_volume_prices_attributes_0_name', with: '5 pieces discount'
-    select 'Total price', from: 'variant_volume_prices_attributes_0_discount_type'
+    select 'Total price (All items)', from: 'variant_volume_prices_attributes_0_discount_type'
     fill_in 'variant_volume_prices_attributes_0_range', with: '1..5'
     fill_in 'variant_volume_prices_attributes_0_amount', with: '1'
     click_on 'Update'

spec/models/solidus_volume_pricing/pricer_spec.rb

@@ -222,6 +222,55 @@
       end
     end
 
+    context 'discount_type is banded' do
+      before do
+        variant.volume_prices.create!(amount: 7, discount_type: 'price', range: '(10...20)')
+        variant.volume_prices.create!(amount: 4, discount_type: 'banded_dollar', range: '(20...30)')
+        variant.volume_prices.create!(amount: 0.5, discount_type: 'banded_percent', range: '(30...40)')
+        variant.volume_prices.create!(amount: 1, discount_type: 'banded_price', range: '(40+)')
+      end
+
+      context 'when quantity does not match the range' do
+        it_behaves_like 'having the variant price'
+      end
+
+      context 'when quantity matches the first (dollar) band' do
+        let(:quantity) { 25 }
+
+        it 'uses discount based calculation, but for the quantity inside the band only' do
+          # Pre-band: 19 * 7.00 +
+          #   Band 1: 6 * (10.00 - 4.00)
+          # Divided by 25
+          expect(subject).to eq('$6.76')
+        end
+      end
+
+      context 'when quantity matches the second (percent) band' do
+        let(:quantity) { 35 }
+
+        it 'uses percent based calculation, but for the quantity inside the band only' do
+          # Pre-band: 19 * 7.00 +
+          #   Band 1: 10 * (10.00 - 4.00) +
+          #   Band 2: 6 * (10.00 * 0.5)
+          # Divided by 35
+          expect(subject).to eq('$6.37')
+        end
+      end
+
+      context 'when quantity matches the third (price) band' do
+        let(:quantity) { 45 }
+
+        it 'uses the set volume price, but for the quantity inside the band only' do
+          # Pre-band: 19 * 7.00 +
+          #   Band 1: 10 * (10.00 - 4.00) +
+          #   Band 2: 10 * (10.00 * 0.5) +
+          #   Band 3: 6 * 1.00
+          # Divided by 45
+          expect(subject).to eq('$5.53')
+        end
+      end
+    end
+
     context 'discount_type is unknown' do
       before do
         variant.volume_prices.create(amount: 7, discount_type: 'foo', range: '(10+)')

spec/models/spree/volume_price_spec.rb

@@ -11,7 +11,10 @@
   it { is_expected.to validate_presence_of(:discount_type) }
   it { is_expected.to validate_presence_of(:amount) }
 
-  it { is_expected.to validate_inclusion_of(:discount_type).in_array(%w[price dollar percent]) }
+  it {
+    expect(subject).to validate_inclusion_of(:discount_type).in_array(%w[price dollar percent banded_price banded_dollar
+                                                                         banded_percent])
+  }
 
   describe '.for_variant' do
     subject { described_class.for_variant(variant, user: user) }