mongodb/mongoose的本地数据库链接不上，shell里的提示如下，这是什么原因？

Error: failed to connect to [127.0.0.1:27017]
    at null.<anonymous> (/home/alfred/housemap1.0/node_modules/session-mongoose/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/server.js:540:74)
    at EventEmitter.emit (events.js:106:17)
    at null.<anonymous> (/home/alfred/housemap1.0/node_modules/session-mongoose/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:140:15)
    at EventEmitter.emit (events.js:98:17)
    at Socket.<anonymous> (/home/alfred/housemap1.0/node_modules/session-mongoose/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/connection.js:478:10)
    at Socket.EventEmitter.emit (events.js:95:17)
    at net.js:441:14
    at process._tickCallback (node.js:415:13)