[{_id:1,players:[{name:"George",roles:["GUITAR"]},{name:"John",roles:["SINGER","GUITAR"]},{name:"Paul",roles:["SINGER","BASS GUITAR"]},{name:"Ringo",roles:["DRUMS"]},]}]

db.collection.aggregate([{$unwind:"$players"},{$unwind:"$players.roles"},{$match:{"players.roles":"SINGER"}},{$group:{_id:"$_id",cnt:{$sum:1}}},{$match:{cnt:{$gt:1}}}])