class DbCollection{
 Db db;
 String collectionName;
 DbCollection(this.db, this.collectionName){}
 String fullName() => "${db.databaseName}.$collectionName";
 Future save(Map document, {WriteConcern writeConcern}){
   var id;
   bool createId = false;
   if (document.containsKey("_id")){
     id = document["_id"];
     if (id == null){
       createId = true;
     }
   }
   if (id != null){
     return update({"_id": id}, document, writeConcern: writeConcern);
   }
   else{
     if (createId) {
       document["_id"] = new ObjectId();
     }
     return insert(document, writeConcern: writeConcern);
   }
 }
Future insertAll(List<Map> documents, {WriteConcern writeConcern}){
   MongoInsertMessage insertMessage = new MongoInsertMessage(fullName(),documents);
   db.executeMessage(insertMessage, writeConcern);
   return db._getAcknowledgement(writeConcern: writeConcern);
 }
 Future update(selector, document, {bool upsert: false, bool multiUpdate: false, WriteConcern writeConcern}){
   int flags = 0;
   if (upsert) {
     flags |= 0x1;
   }
   if (multiUpdate) {
     flags |= 0x2;
   }

   MongoUpdateMessage message = new MongoUpdateMessage(fullName(), 
       _selectorBuilder2Map(selector), document, flags);
   db.executeMessage(message, writeConcern);
   return db._getAcknowledgement(writeConcern: writeConcern);
 }

/**
 * Creates a cursor for a query that can be used to iterate over results from MongoDB
 * ##[selector]
 * parameter represents query to locate objects. If omitted as in `find()` then query matches all documents in colleciton.
 * Here's a more selective example:
 *     find({'last_name': 'Smith'})
 * Here our selector will match every document where the last_name attribute is 'Smith.'
 *
 */
 Cursor find([selector]) {
   return new Cursor(db, this, selector);
 }

 Future<Map> findOne([selector]){
   Cursor cursor = new Cursor(db, this, selector);
   Future<Map> result = cursor.nextObject();
   cursor.close();
   return result;
 }
 Future drop() => db.dropCollection(collectionName);
 Future remove([selector, WriteConcern writeConcern]) => db.removeFromCollection(collectionName, _selectorBuilder2Map(selector), writeConcern);
 Future<int> count([selector]){
   Completer completer = new Completer();
   db.executeDbCommand(DbCommand.createCountCommand(db,collectionName,_selectorBuilder2Map(selector))).then((reply){
     completer.complete(reply["n"].toInt());
   });
   return completer.future;
 }
 
 Future distinct(String field, [selector]) =>
   db.executeDbCommand(DbCommand.createDistinctCommand(db,collectionName,field,_selectorBuilder2Map(selector)));
 
 Future aggregate(List pipeline){
   var cmd = DbCommand.createAggregateCommand(db,collectionName,pipeline);
   return db.executeDbCommand(cmd);
 }
 
 Future insert(Map document, {WriteConcern writeConcern}) => insertAll([document], writeConcern: writeConcern);

 Map _selectorBuilder2Map(selector) {
   if (selector == null) {
     return {};
   }
   if (selector is SelectorBuilder) {
     return selector.map['\$query'];
   }
   return selector;
 }
}