Fixing association bug to avoid searching when foreign key is nil

lib/couchrest/model/associations.rb

@@ -109,7 +109,7 @@ module CouchRest
           base = options[:proxy] || options[:class_name]
           class_eval <<-EOS, __FILE__, __LINE__ + 1
             def #{attrib}
-              @#{attrib} ||= #{base}.get(self.#{options[:foreign_key]})
+              @#{attrib} ||= #{options[:foreign_key]}.nil? ? nil : #{base}.get(self.#{options[:foreign_key]})
             end
           EOS
         end

spec/couchrest/assocations_spec.rb

@@ -62,6 +62,12 @@ describe "Assocations" do
       @invoice.client_id.should be_nil
     end
 
+    it "should not try to search for association if foreign_key is nil" do
+      @invoice.client_id = nil
+      Client.should_not_receive(:get)
+      @invoice.client
+    end
+
     it "should raise error if class name does not exist" do
       lambda {
         class TestBadAssoc < CouchRest::Model::Base