In some queries to the search server, elements representing core entities are returned without an accompanying id attribute. For an example, see the release-group elements in:

http://musicbrainz.org/ws/2/recording?query=quiz

This makes access to the API inefficient, as obtaining further information about the release-group first requires a request to the API about the parent release in order to obtain the id for the release-group, before then being able to make another request to the API to obtain information about the release-group itself.

This missing attribute doesn't violate the schema[1] as it currently stands, which lists all id attributes as optional. However, I can't see why the id attribute needs to be optional, and would far prefer that the schema were amended to make the id attribute mandatory.

[1] http://svn.musicbrainz.org/mmd-schema/trunk/schema/musicbrainz_mmd-2.0.rng