Remove expanded instance variables after a resource has been updated (stripe#1698)

* Fix object-as-hash issue

* Switch to removing member variables

* Clarify test case

Author: mbroshi-stripe

lib/stripe/stripe_object.rb

@@ -313,6 +313,7 @@ class << self; self; end
     protected def remove_accessors(keys)
       # not available in the #instance_eval below
       protected_fields = self.class.protected_fields
+      obj = self # capture self to use inside instance_eval
 
       metaclass.instance_eval do
         keys.each do |k|
@@ -342,6 +343,11 @@ class << self; self; end
                    "collide with an API property name.")
             end
           end
+
+          # Also remove instance variables so that static attr_readers (if any exist)
+          # will return nil instead of stale data or data of the wrong type
+          ivar_name = :"@#{k}"
+          obj.remove_instance_variable(ivar_name) if obj.instance_variable_defined?(ivar_name)
         end
       end
     end

test/stripe/invoice_payments_test.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+require File.expand_path("../test_helper", __dir__)
+
+module Stripe
+  class InvoicePaymentsTest < Test::Unit::TestCase
+    context "invoice.payments field" do
+      should "return nil after instance method calls when field not in response" do
+        # Simulate invoice.retrieve() with payments field expanded
+        stub_request(:get, "#{Stripe.api_base}/v1/invoices/in_123")
+          .to_return(body: JSON.generate({
+            id: "in_123",
+            object: "invoice",
+            amount_paid: 0,
+            payments: {
+              object: "list",
+              data: [{
+                id: "invpay_123",
+                object: "invoice.payment",
+                status: "open",
+              }],
+              has_more: false,
+              url: "/v1/invoices/in_123/payments",
+            },
+          }))
+
+        # Simulate invoice.pay(), which returns invoice WITHOUT payments field
+        stub_request(:post, "#{Stripe.api_base}/v1/invoices/in_123/pay")
+          .to_return(body: JSON.generate({
+            id: "in_123",
+            object: "invoice",
+            amount_paid: 0,
+            status: "paid",
+            # NOTE: payments field is NOT included in response
+          }))
+
+        invoice = Stripe::Invoice.retrieve({ id: "in_123", expand: ["payments"] })
+
+        assert_not_nil invoice.payments, "payments should be non-nil before pay"
+        invoice.pay
+        assert_nil invoice.payments, "payments should be nil after pay when not in response"
+      end
+    end
+  end
+end