Pass params to transfer reversal

lib/stripe/api_operations/nested_resource.rb

@@ -35,6 +35,7 @@ def nested_resource_class_methods(resource, path: nil, operations: nil,
         end
       end
 
+      # rubocop:disable Metrics/MethodLength
       private def define_operation(
         resource,
         operation,
@@ -53,12 +54,30 @@ def nested_resource_class_methods(resource, path: nil, operations: nil,
             )
           end
         when :retrieve
+          # TODO: (Major) Split params_or_opts to params and opts and get rid of the complicated way to add params
           define_singleton_method(:"retrieve_#{resource}") \
-              do |id, nested_id, opts = {}|
+              do |id, nested_id, params_or_opts = {}, definitely_opts = nil|
+            opts = nil
+            params = nil
+            if definitely_opts.nil?
+              unrecognized_key = params_or_opts.keys.find { |k| !Util::OPTS_USER_SPECIFIED.include?(k) }
+              if unrecognized_key
+                raise ArgumentError,
+                      "Unrecognized request option: #{unrecognized_key}. Did you mean to specify this as " \
+                      "retrieve params? " \
+                      "If so, you must explicitly pass an opts hash as a third argument. " \
+                      "For example: .retrieve({#{unrecognized_key}: 'foo'}, {})"
+              end
+
+              opts = params_or_opts
+            else
+              opts = definitely_opts
+              params = params_or_opts
+            end
             request_stripe_object(
               method: :get,
               path: send(resource_url_method, id, nested_id),
-              params: {},
+              params: params,
               opts: opts
             )
           end
@@ -96,6 +115,7 @@ def nested_resource_class_methods(resource, path: nil, operations: nil,
           raise ArgumentError, "Unknown operation: #{operation.inspect}"
         end
       end
+      # rubocop:enable Metrics/MethodLength
     end
   end
 end

test/stripe/transfer_test.rb

@@ -84,5 +84,16 @@ class TransferTest < Test::Unit::TestCase
         assert reversals.data.is_a?(Array)
       end
     end
+
+    should "retrieve a reversal with expand" do
+      reversal = Stripe::Transfer.retrieve_reversal(
+        "tr_123",
+        "trr_123",
+        { expand: %w[transfer] },
+        {}
+      )
+      assert_requested :get, "#{Stripe.api_base}/v1/transfers/tr_123/reversals/trr_123?expand%5B%5D=transfer"
+      assert reversal.is_a?(Stripe::Reversal)
+    end
   end
 end