remove some lasigns

Author: tenderlove

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,