[{"foo":"a","wwo":"woo1",},{"foo":"a","wwo":"woo2",},{"foo":"a","wwo":"wooN",},]

db.collection.aggregate([{$facet:{missing:[{$group:{_id:"$foo",woo:{$addToSet:"$wwo"}}},{$project:{_id:0,missing_woo:{$setDifference:[["woo1","woo2","wooN","missing_woo1","missing_woo2","missing_wooN"],"$woo"]}}},{$unwind:"$missing_woo"}]}}])