plaha_anton

var map = {};
for (var categoryId in db_obj) {
    if (!db_obj.hasOwnProperty(categoryId)) continue;
    var entry = db_obj[categoryId];
    for(var userId in entry) {
        if(!entry.hasOwnProperty(userId)) continue;
        if(!map.hasOwnProperty(userId)) {
             map[userId] = [];
        }
        map[userId].push(categoryId);
    }
}