[{_id:"5e2b8a2065318f95166deedc",expenses:[{amount:100},{amount:200}]},{_id:"5e2b8a2065318f95166deedd",expenses:[]},{_id:"5e2b8a2065318f95166deede",expenses:[{amount:400},{amount:600}]}]

db.collection.aggregate([{$match:{"expenses.1":{$exists:true}}}])