This returns an empty document rather than null/nothing :
db.test.findAndModify({query:

, update : {$set : { a:2}}, fields:

, upsert:true} )

This returns a document with only the _id set rather than the entire newly inserted document :
db.test.findAndModify({query:

, update : {$set : { a:2}}, fields:

, upsert:true, new: true } )

Is this intended? If so, is it useful to open a JIRA issue to change this?