class MongoDatabase {
 Db _db;
 Future _conn;
 List<Future> init = [];
 DbCollection _lock;
 Cache cache;

 MongoDatabase(String url, {Cache this.cache: dummyCache} ) {
   _db = new Db(url);
   _conn = _db.open(); // open connection to database
   init.add(_conn);
   init.add(_conn.then((_) {
     _lock = _db.collection(LOCK_COLLECTION_NAME);
     return true;
     }));
 }

 void close() {
   Future.wait(init).then((_) => _db.close());
 }

 void create_collection(String collectionName) {
   init.add(_conn.then((_) =>
       _db.createIndex(historyCollectionName(collectionName), key: 'version',
       unique: true)));
 }

 /**
  * Creates index on chosen collection and corresponding indexes on collection
  * history. keys is a map in form {field_name: 1 or -1} with 1/-1 specifying
  * ascending/descending order (same as the map passed to mongo function
  * ensureIndex).
  */
 void createIndex(String collectionName, Map keys, {unique: false}) {
   Map beforeKeys = {};
   Map afterKeys = {};
   keys.forEach((key, val) {
     beforeKeys['before.$key'] = val;
     afterKeys['after.$key'] = val;
   });
   beforeKeys['version'] = 1;
   afterKeys['version'] = 1;
   init.add(_conn.then((_) =>
       _db.createIndex(historyCollectionName(collectionName),
           keys: beforeKeys)));
   init.add(_conn.then((_) =>
       _db.createIndex(historyCollectionName(collectionName),
           keys: afterKeys)));
   if (keys.isNotEmpty) {
     init.add(_conn.then((_) =>
         _db.createIndex(collectionName, keys: keys, unique: unique)));
   }
 }

 MongoProvider collection(String collectionName) {
   DbCollection collection = _db.collection(collectionName);
   DbCollection collectionHistory =
       _db.collection(historyCollectionName(collectionName));
   return new MongoProvider(collection, collectionHistory, _lock, cache);
 }

 Future dropCollection(String collectionName) =>
   _conn.then((_) => Future.wait([
     _db.collection(collectionName).drop(),
     _db.collection(historyCollectionName(collectionName)).drop()
   ]));

 Future removeLocks() => _lock.drop();
}