diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 75ae06f5e95eb8d9659c6644722b7568f37bee8d..a10e94d5f703365a61efe462dd9b0ed1f9ed88d5 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -24,14 +24,14 @@ def reset_primary_key #:nodoc:
         end
 
         def get_primary_key(base_name) #:nodoc:
-          key = 'id'
           case primary_key_prefix_type
-            when :table_name
-              key = base_name.to_s.foreign_key(false)
-            when :table_name_with_underscore
-              key = base_name.to_s.foreign_key
+          when :table_name
+            base_name.to_s.foreign_key(false)
+          when :table_name_with_underscore
+            base_name.to_s.foreign_key
+          else
+            'id'
           end
-          key
         end
 
         # Sets the name of the primary key column to use to the given value,