[{_id:ObjectId("65708c6237b62067296a398f"),date:"2013-10-21T13:28:06.419z",description:"playstation",category:"miscellaneous",cost:499,users:[{username:"Giacomo",amount:499}]},{_id:ObjectId("6570950cecb7eb1b4b868409"),date:"2013-10-21T13:28:06.419z",description:"tennis court",category:"sport",cost:100,users:[{username:"prova",amount:50},{username:"Giacomo",amount:50}]},{_id:ObjectId("6570953aecb7eb1b4b86840a"),date:"2013-10-21T13:28:06.419z",description:"netflix subscription",category:"entertainment",cost:100,users:[{username:"prova",amount:10}]}]

db.collection.aggregate({"$match":{"users":{$elemMatch:{username:"Giacomo"}}}})