[{"name":"tizio","surname":"semproni","birthday":"01/02/1923","home":{"road":"via","roadname":"bianca","roadN":12,"city":"rome","country":"italy"}},{"name":"caio","surname":"giulio","birthday":"02/03/1932","home":{"road":"via","roadname":"rossa","roadN":21,"city":"milan","country":"italy"}},{"name":"mario","surname":"rossi","home":{"road":"via","roadname":"della pace","roadN":120,"city":"rome","country":"italy"}}]

db.collection.aggregate([{$project:{_id:0,name:1,surname:1,birthday:1,address:{$reduce:{input:{$objectToArray:"$home"},initialValue:"",in:{$concat:["$$value",{$cond:[{$eq:["$$value",""]},""," "]},{$toString:"$$this.v"}]}}}}}])