Objects with lazy loaded fields trigger additional queries when db_cache is enabled #183

As described by @davidpede in #152, when an object is loaded with lazy-loaded fields, additional queries are fired automatically to load the lazy fields if the db_cache is enabled. This implies that the signature for storing cached entries is insufficient to cache objects loaded this way. Two possible solutions come to mind. We would either need to include the selected (or unselected) columns in the cache signature, or we need to prevent caching of objects that have fields that were not selected.