Do not add record to identity map if the record doesn't have values f…

…or all the columns, so we don't get 'MissingAttributeError' later when trying to access other fields of the same record.

@@ -90,7 +90,7 @@ def get(klass, primary_key)
       end
 
       def add(record)
-        repository[record.class.symbolized_sti_name][record.id] = record
+        repository[record.class.symbolized_sti_name][record.id] = record if record.class.column_names - record.attribute_names == []
       end
 
       def remove(record)