Currently #only works on a document's fields and on embedded associations, but not on referenced associations. https://jira.mongodb.org/browse/MONGOID-2681 makes projections correctly propagate into the embedded associations. For referenced associations, Mongoid currently doesn't project (hence retrieves all fields). This is not a major issue in the sense that the application behavior remains correct, just more data is fetched than necessary. However, we can investigate what would be needed to make #only work across referenced associations.