code: function getConnection(connectionString) {
        return new Promise(function(resolve, reject) {
          // If connectionString is null or undefined, return an error.
          if (_.isEmpty(connectionString)) {
            return reject('getConnection must be called with a mongo connection string');
          }
  
          // Check if a connection already exists for the provided connectionString.
          var pool = _.findWhere(connections, { connectionString: connectionString });
  
          // If a connection pool was found, resolve the promise with it.
          if (pool) {
            return resolve(pool.db);
          }
  
          // If the connection pool has not been instantiated,
          // instantiate it and return the connection.
          MongoClient.connect(connectionString, function(err, database) {
            if (err) {
              return reject(err);
            }
  
            // Store the connection in the connections array.
            connections.push({
              connectionString: connectionString,
              db: database
            });
  
            return resolve(database);
          });
        });
      }
docs: Gets a Mongo connection from the pool.
  
  If the connection pool has not been instantiated yet, it is first
  instantiated and a connection is returned.
  
  @returns {Promise|Db} - A promise object that resolves to a Mongo db object.