[{_id:"1",text:"xxxxxxxc",choices:{a:["xxx","yyy","zzz"],b:["aaa","bbb","ccc"],c:["lll","mmm","ddd"],}}]

db.collection.aggregate([{$addFields:{"c":{"$objectToArray":"$choices"}}},{$match:{"c.v":"yyy"}},{"$project":{c:0}}])