account_id isn't always required. It may be blank for :subscribe calls.

lib/zuora/objects/contact.rb

@@ -2,7 +2,8 @@ module Zuora::Objects
   class Contact < Base
     belongs_to :account
 
-    validates_presence_of :account_id, :first_name, :last_name
+    validates_presence_of :first_name, :last_name
+    validates_presence_of :account_id, :unless => Proc.new { |contact| contact.new_record? }
     validates_length_of :first_name, :maximum => 100
     validates_length_of :last_name, :maximum => 100
     validates_length_of :nick_name, :maximum => 100, :allow_nil => true

lib/zuora/objects/payment_method.rb

@@ -2,7 +2,7 @@ module Zuora::Objects
   class PaymentMethod < Base
     belongs_to :account
 
-    validates_presence_of :account_id
+    validates_presence_of :account_id, :unless => Proc.new { |contact| contact.new_record? }
 
     # Generic Validations
     validates_inclusion_of    :type, :in => %w(ACH Cash Check CreditCard CreditCardReferenceTransaction DebitCard Other PayPal WireTransfer)

spec/zuora/objects/contact_spec.rb

@@ -20,17 +20,31 @@
       subject.should_not be_valid
     end
 
-    it "requires account_id" do
-      subject.errors[:account_id].should include("can't be blank")
-    end
-
     it "requires first_name" do
       subject.errors[:first_name].should include("can't be blank")
     end
 
     it "requires last_name" do
       subject.errors[:last_name].should include("can't be blank")
     end
+
+    context "when new record" do
+      it "should not require account_id" do
+        subject.errors[:account_id].should_not include("can't be blank")
+      end
+    end
+
+    context "when persisted record" do
+      before :each do
+        subject.stub(:new_record? => false)
+
+        subject.should_not be_valid
+      end
+
+      it "requires account_id" do
+        subject.errors[:account_id].should include("can't be blank")
+      end
+    end
   end
 
   describe "read only attributes" do