Backports minimum quantity promotion rule from Solidus

Original PR: solidusio/solidus#5452

This new rule allows for an easy way of providing "bulk" discounts
through the promotion system. It will prevent a promotion from being
applied until it meets a quantity threshold. The rule will also account
for other rules that limit the applicable line items. (e.g.: Taxons,
options values, etc.)

Co-authored-by: Benjamin Wil <[email protected]>
Co-authored-by: Adam Mueller <[email protected]>

app/models/solidus_friendly_promotions/rules/minimum_quantity.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+module SolidusFriendlyPromotions
+  module Rules
+    # Promotion rule for ensuring an order contains a minimum quantity of
+    # applicable items.
+    #
+    # This promotion rule is only compatible with the "all" match policy. It
+    # doesn't make a lot of sense to use it without that policy as it reduces
+    # it to a simple quantity check across the entire order which would be
+    # better served by an item total rule.
+    class MinimumQuantity < PromotionRule
+      include OrderLevelRule
+
+      validates :preferred_minimum_quantity, numericality: {only_integer: true, greater_than: 0}
+
+      preference :minimum_quantity, :integer, default: 1
+
+      # Will look at all of the "applicable" line items in the order and
+      # determine if the sum of their quantity is greater than the minimum.
+      #
+      # "Applicable" items are ones that pass all eligibility checks of applicable rules.
+      #
+      # When false is returned, the reason will be included in the
+      # `eligibility_errors` object.
+      #
+      # @param order [Spree::Order] the order we want to check eligibility on
+      # @return [Boolean] true if promotion is eligible, false otherwise
+      def eligible?(order)
+        applicable_line_items = order.line_items.select do |line_item|
+          promotion.rules.select do |rule|
+            rule.applicable?(line_item)
+          end.all? { _1.eligible?(line_item) }
+        end
+
+        if applicable_line_items.sum(&:quantity) < preferred_minimum_quantity
+          eligibility_errors.add(
+            :base,
+            eligibility_error_message(:quantity_less_than_minimum, count: preferred_minimum_quantity),
+            error_code: :quantity_less_than_minimum
+          )
+        end
+
+        eligibility_errors.empty?
+      end
+    end
+  end
+end

app/views/solidus_friendly_promotions/admin/promotion_rules/rules/_minimum_quantity.html.erb

@@ -0,0 +1,5 @@
+<div class="field">
+  <% field_name = "#{param_prefix}[preferred_minimum_quantity]" %>
+  <%= label_tag field_name, promotion_rule.model_name.human %>
+  <%= number_field_tag field_name, promotion_rule.preferred_minimum_quantity, class: "fullwidth", min: 1 %>
+</div>

config/locales/en.yml

@@ -91,6 +91,10 @@ en:
       solidus_friendly_promotions/rules/user_logged_in:
         no_user_specified: You need to login before applying this coupon code.
       solidus_friendly_promotions/rules/user_role:
+      solidus_friendly_promotions/rules/minimum_quantity:
+        quantity_less_than_minimum:
+          one: "You need to add a least 1 applicable item to your order."
+          other: "You need to add a least %{count} applicable items to your order."
     product_rule:
       choose_products: Choose products
       label: Order must contain %{select} these products
@@ -157,6 +161,7 @@ en:
       solidus_friendly_promotions/rules/item_total: Item Total
       solidus_friendly_promotions/rules/discounted_item_total: Item Total after previous lanes
       solidus_friendly_promotions/rules/landing_page: Landing Page
+      solidus_friendly_promotions/rules/minimum_quantity: Minimum Quantity
       solidus_friendly_promotions/rules/nth_order: Nth Order
       solidus_friendly_promotions/rules/one_use_per_user: One Use Per User
       solidus_friendly_promotions/rules/option_value: Option Value(s)
@@ -201,6 +206,8 @@ en:
         preferred_line_item_applicable: Should also apply to line items
       solidus_friendly_promotions/rules/line_item_option_value:
         description: Line Item has specified product with matching option value
+      solidus_friendly_promotions/rules/minimum_quantity:
+        description: Order contains minimum quantity of applicable items
       solidus_friendly_promotions/rules/product:
         description: Order includes specified product(s)
         line_item_level_description: 'Line item matches the specified products:'

lib/generators/solidus_friendly_promotions/install/templates/initializer.rb

@@ -84,6 +84,7 @@
     "SolidusFriendlyPromotions::Rules::FirstRepeatPurchaseSince",
     "SolidusFriendlyPromotions::Rules::ItemTotal",
     "SolidusFriendlyPromotions::Rules::DiscountedItemTotal",
+    "SolidusFriendlyPromotions::Rules::MinimumQuantity",
     "SolidusFriendlyPromotions::Rules::NthOrder",
     "SolidusFriendlyPromotions::Rules::OneUsePerUser",
     "SolidusFriendlyPromotions::Rules::OptionValue",

spec/models/solidus_friendly_promotions/rules/minimum_quantity_spec.rb

@@ -0,0 +1,108 @@
+# frozen_string_literal: true
+
+RSpec.describe SolidusFriendlyPromotions::Rules::MinimumQuantity do
+  subject(:quantity_rule) { described_class.new(preferred_minimum_quantity: 2) }
+
+  describe "#valid?" do
+    let(:promotion) { build(:friendly_promotion) }
+
+    before { promotion.rules << quantity_rule }
+
+    it { is_expected.to be_valid }
+
+    context "when minimum quantity is zero" do
+      subject(:quantity_rule) { described_class.new(preferred_minimum_quantity: 0) }
+
+      it { is_expected.not_to be_valid }
+    end
+  end
+
+  describe "#applicable?" do
+    subject { quantity_rule.applicable?(promotable) }
+
+    context "when promotable is an order" do
+      let(:promotable) { Spree::Order.new }
+
+      it { is_expected.to be true }
+    end
+
+    context "when promotable is a line item" do
+      let(:promotable) { Spree::LineItem.new }
+
+      it { is_expected.to be false }
+    end
+  end
+
+  describe "#eligible?" do
+    subject { quantity_rule.eligible?(order) }
+
+    let(:order) do
+      create(
+        :order_with_line_items,
+        line_items_count: line_items.length,
+        line_items_attributes: line_items
+      )
+    end
+    let(:promotion) { build(:friendly_promotion) }
+
+    before { promotion.rules << quantity_rule }
+
+    context "when only the quantity rule is applied" do
+      context "when the quantity is less than the minimum" do
+        let(:line_items) { [{quantity: 1}] }
+
+        it { is_expected.to be false }
+      end
+
+      context "when the quantity is equal to the minimum" do
+        let(:line_items) { [{quantity: 2}] }
+
+        it { is_expected.to be true }
+      end
+
+      context "when the quantity is greater than the minimum" do
+        let(:line_items) { [{quantity: 4}] }
+
+        it { is_expected.to be true }
+      end
+    end
+
+    context "when another rule limits the applicable items" do
+      let(:carry_on) { create(:variant) }
+      let(:other_carry_on) { create(:variant) }
+      let(:everywhere_bag) { create(:product).master }
+
+      let(:product_rule) {
+        SolidusFriendlyPromotions::Rules::LineItemProduct.new(
+          products: [carry_on.product, other_carry_on.product],
+          preferred_match_policy: "any"
+        )
+      }
+
+      before { promotion.rules << product_rule }
+
+      context "when the applicable quantity is less than the minimum" do
+        let(:line_items) do
+          [
+            {variant: carry_on, quantity: 1},
+            {variant: everywhere_bag, quantity: 1}
+          ]
+        end
+
+        it { is_expected.to be false }
+      end
+
+      context "when the applicable quantity is greater than the minimum" do
+        let(:line_items) do
+          [
+            {variant: carry_on, quantity: 1},
+            {variant: other_carry_on, quantity: 1},
+            {variant: everywhere_bag, quantity: 1}
+          ]
+        end
+
+        it { is_expected.to be true }
+      end
+    end
+  end
+end