Don't call self.class unless necessary. Closes #3171.

activerecord/lib/active_record/reflection.rb

@@ -212,8 +212,8 @@ def association_foreign_key
       end
 
       # klass option is necessary to support loading polymorphic associations
-      def association_primary_key(klass = self.klass)
-        options[:primary_key] || klass.primary_key
+      def association_primary_key(klass = nil)
+        options[:primary_key] || (klass || self.klass).primary_key
       end
 
       def active_record_primary_key

activerecord/test/cases/reflection_test.rb

@@ -244,6 +244,7 @@ def test_association_primary_key
     # Normal association
     assert_equal "id",   Author.reflect_on_association(:posts).association_primary_key.to_s
     assert_equal "name", Author.reflect_on_association(:essay).association_primary_key.to_s
+    assert_equal "name", Essay.reflect_on_association(:writer).association_primary_key.to_s
 
     # Through association (uses the :primary_key option from the source reflection)
     assert_equal "nick", Author.reflect_on_association(:subscribers).association_primary_key.to_s