Add Comparable to Spree::Money

This commit includes Comparable and implements `<=>`. Including
Comparable also gives us some basic comparison methods like `<`, `>`,
`.between?`, and `sort`.

We need to add an additional currency comparison because `::Money` will
automatically try to convert the second currency in order to be able to
compare it with the first. We don't want this to happen so we're going
to explicitly raise an error.

CHANGELOG.md

@@ -1,15 +1,21 @@
 ## Solidus 2.2.0 (master, unreleased)
 
 *   Promotion and Shipping calculators can be created or have their type
-    changed without saving and reloading the page. [#1618](https://github.com/solidusio/solidus/pull/1618)
+    changed without saving and reloading the page.
+    [#1618](https://github.com/solidusio/solidus/pull/1618)
 
-*   Product slugs no longer have a minimum length validation to match the Product name validation.
+*   Product slugs no longer have a minimum length validation to match the
+    Product name validation.
     [#1616](https://github.com/solidusio/solidus/pull/1616)
 
 *   Changed the promotion field in the admin order search to only find orders
     which used the specified promotion code, instead of any code on an applied
     promotion. [#1662](https://github.com/solidusio/solidus/pull/1662)
 
+*   `Spree::Money` now includes `Comparable` and the `<=>` operator for
+    comparisons. A comparison will fail if the two objects do not use the same
+    currency. [#1682](https://github.com/solidusio/solidus/pull/1682)
+
 ## Solidus 2.1.0 (unreleased)
 
 *   The OrderUpdater (as used by `order.update!`) now fully updates taxes.
@@ -83,7 +89,7 @@
     For now `PromotionAction` provides a default remove_from method, with a
     deprecation warning that subclasses should define their own remove_from
     method.
-    
+
     [#1451](https://github.com/solidusio/solidus/pull/1451)
 
 *   Remove `is_default` boolean from `Spree::Price` model

core/lib/spree/money.rb

@@ -6,6 +6,8 @@ module Spree
   # Spree::Money is a relatively thin wrapper around Monetize which handles
   # formatting via Spree::Config.
   class Money
+    include Comparable
+    DifferentCurrencyError = Class.new(StandardError)
     RUBY_NUMERIC_STRING = /\A-?\d+(\.\d+)?\z/
 
     class <<self
@@ -92,6 +94,22 @@ def as_json(*)
       to_s
     end
 
+    def <=>(other)
+      if !other.respond_to?(:money)
+        raise TypeError, "Can't compare #{other.class} to Spree::Money"
+      end
+      if self.currency != other.currency
+        # By default, ::Money will try to run a conversion on `other.money` and
+        # try a comparison on that. We do not want any currency conversion to
+        # take place so we'll catch this here and raise an error.
+        raise(
+          DifferentCurrencyError,
+          "Can't compare #{self.currency} with #{other.currency}"
+        )
+      end
+      @money <=> other.money
+    end
+
     # Delegates comparison to the internal ruby money instance.
     #
     # @see http://www.rubydoc.info/gems/money/Money/Arithmetic#%3D%3D-instance_method

core/spec/lib/spree/money_spec.rb

@@ -268,4 +268,36 @@
       end
     end
   end
+
+  it "includes Comparable" do
+    expect(described_class).to include(Comparable)
+  end
+
+  describe "<=>" do
+    let(:usd_10) { Spree::Money.new(10, currency: "USD") }
+    let(:usd_20) { Spree::Money.new(20, currency: "USD") }
+    let(:usd_30) { Spree::Money.new(30, currency: "USD") }
+
+    it "compares the two amounts" do
+      expect(usd_20 <=> usd_20).to eq 0
+      expect(usd_20 <=> usd_10).to be > 0
+      expect(usd_20 <=> usd_30).to be < 0
+    end
+
+    context "with a non Spree::Money object" do
+      it "raises an error" do
+        expect { usd_10 <=> 20 }.to raise_error(TypeError)
+      end
+    end
+
+    context "with differing currencies" do
+      let(:cad) { Spree::Money.new(10, currency: "CAD") }
+
+      it "raises an error" do
+        expect { usd_10 <=> cad }.to raise_error(
+          Spree::Money::DifferentCurrencyError
+        )
+      end
+    end
+  end
 end