Add RateStore#marshal_dump so a bank using it can be marshaled

Money::Bank::VariableExchange#marshal_load rebuilds the store with
store_info.shift.new(*store_info), which requires the store's
#marshal_dump to return [StoreClass, *constructor_args]. Without it,
marshaling a bank whose store is a RateStore fails, which happens when
the configured default bank is cached under a Marshal-based serializer.

RateStore takes no constructor arguments, so returning [self.class] is
enough to rebuild it on load.

Author: jtarchie

CHANGELOG.md

@@ -8,6 +8,9 @@ This project adheres to [Semantic Versioning].
 Changes:
 - Drop support for Rails < 7.0.
 
+Fix:
+- Make `RateStore` marshalable so a bank using it can be marshaled.
+
 ## v1.4.1
 
 Fix:

lib/active_currency/rate_store.rb

@@ -3,5 +3,15 @@
 module ActiveCurrency
   class RateStore < DatabaseStore
     include CacheableStore
+
+    # Money::Bank::VariableExchange#marshal_load rebuilds the store with
+    # `store_info.shift.new(*store_info)`, so a store's #marshal_dump must
+    # return `[StoreClass, *constructor_args]`. RateStore takes no arguments,
+    # so returning `[self.class]` lets a bank backed by this store be
+    # marshaled -- for example when the configured default bank is cached
+    # under a Marshal-based cache/serializer.
+    def marshal_dump
+      [self.class]
+    end
   end
 end

spec/lib/active_currency/rate_store_spec.rb

@@ -104,4 +104,18 @@
       end
     end
   end
+
+  describe "#marshal_dump" do
+    it "dumps the class so the store can be rebuilt on load" do
+      expect(store.marshal_dump).to eq([described_class])
+    end
+
+    it "lets a bank backed by the store round-trip through Marshal" do
+      bank = Money::Bank::VariableExchange.new(store)
+
+      loaded = Marshal.load(Marshal.dump(bank))
+
+      expect(loaded.store).to be_a(described_class)
+    end
+  end
 end