Connections

You can connect to MongoDB with the mongoose.connect() method.

mongoose.connect('mongodb://localhost/myapp');

This is the minimum needed to connect the myapp database running locally on the default port (27017). If the local connection fails then try using 127.0.0.1 instead of localhost. Sometimes issues may arise when the local hostname has been changed.

You can also specify several more parameters in the uri:

mongoose.connect('mongodb://username:password@host:port/database?options...');

See the mongodb connection string spec for more detail.

Operation Buffering

Mongoose lets you start using your models immediately, without waiting for mongoose to establish a connection to MongoDB.

mongoose.connect('mongodb://localhost/myapp');
var MyModel = mongoose.model('Test', new Schema({ name: String }));
// Works
MyModel.findOne(function(error, result) { /* ... */ });

That's because mongoose buffers model function calls internally. This buffering is convenient, but also a common source of confusion. Mongoose will not throw any errors by default if you use a model without connecting.

var MyModel = mongoose.model('Test', new Schema({ name: String }));
// Will just hang until mongoose successfully connects
MyModel.findOne(function(error, result) { /* ... */ });

setTimeout(function() {
  mongoose.connect('mongodb://localhost/myapp');
}, 60000);

To disable buffering, turn off the bufferCommands option on your schema. If you have bufferCommands on and your connection is hanging, try turning bufferCommands off to see if you haven't opened a connection properly.

Options

The connect method also accepts an options object which will be passed on to the underlying driver. All options included here take precedence over options passed in the connection string.

mongoose.connect(uri, options);

The following option keys are available:

 db             - passed to the [underlying driver's db instance](http://mongodb.github.io/node-mongodb-native/2.1/api/Db.html)
 server         - passed to the [underlying driver's server instance(s)](http://mongodb.github.io/node-mongodb-native/2.1/api/Server.html)
 replset        - passed to the [underlying driver's ReplSet instance](http://mongodb.github.io/node-mongodb-native/2.1/api/ReplSet.html)
 user           - username for authentication (if not specified in uri)
 pass           - password for authentication (if not specified in uri)
 auth           - options for authentication
 mongos         - passed to the [underlying driver's mongos options](http://mongodb.github.io/node-mongodb-native/2.1/api/Mongos.html)
 promiseLibrary - sets the [underlying driver's promise library](http://mongodb.github.io/node-mongodb-native/2.1/api/MongoClient.html)

Example:

var options = {
  db: { native_parser: true },
  server: { poolSize: 5 },
  replset: { rs_name: 'myReplicaSetName' },
  user: 'myUserName',
  pass: 'myPassword'
}
mongoose.connect(uri, options);

Note: The server option auto_reconnect is defaulted to true which can be overridden. The db option forceServerObjectId is set to false which cannot be overridden.

See the driver for more information about available options.

Note: If auto_reconnect is on, mongoose will give up trying to reconnect after a certain number of failures. Set the server.reconnectTries and server.reconnectInterval options to increase the number of times mongoose will try to reconnect.

// Good way to make sure mongoose never stops trying to reconnect
mongoose.connect(uri, { server: { reconnectTries: Number.MAX_VALUE } });

Callback

The connect() function also accepts a callback parameter and returns a promise.

mongoose.connect(uri, options, function(error) {
  // Check error in initial connection. There is no 2nd param to the callback.
});

// Or using promises
mongoose.connect(uri, options).then(
  () => { /** ready to use. The `mongoose.connect()` promise resolves to undefined. */ },
  err => { /** handle initial connection error }
);

Connection String Options

Mongoose supports the following options in the connection string.

A note about keepAlive

For long running applications, it is often prudent to enable keepAlive with a number of milliseconds. Without it, after some period of time you may start to see "connection closed" errors for what seems like no reason. If so, after reading this, you may decide to enable keepAlive:

options.server.socketOptions = options.replset.socketOptions = { keepAlive: 120 };
mongoose.connect(uri, options);

Replica Set Connections

To connect to a replica set you pass a comma delimited list of hosts to connect to rather than a single host.

mongoose.connect('mongodb://[username:password@]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]' [, options]);

To connect to a single node replica set, specify the replicaSet option.

mongoose.connect('mongodb://host1:port1/?replicaSet=rsName');

Multi-mongos support

High availability over multiple mongos instances is also supported. Pass a connection string for your mongos instances and set the mongos option to true:

mongoose.connect('mongodb://mongosA:27501,mongosB:27501', { mongos: true }, cb);

Multiple connections

So far we've seen how to connect to MongoDB using Mongoose's default connection. At times we may need multiple connections open to Mongo, each with different read/write settings, or maybe just to different databases for example. In these cases we can utilize mongoose.createConnection() which accepts all the arguments already discussed and returns a fresh connection for you.

var conn = mongoose.createConnection('mongodb://[username:password@]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]', options);

This connection object is then used to create and retrieve models. Models are always scoped to a single connection.

Connection pools

Each connection, whether created with mongoose.connect or mongoose.createConnection are all backed by an internal configurable connection pool defaulting to a size of 5. Adjust the pool size using your connection options:

// single server
var uri = 'mongodb://localhost/test';
mongoose.createConnection(uri, { server: { poolSize: 4 }});

// for a replica set
mongoose.createConnection(uri, { replset: { poolSize: 4 }});

// passing the option in the URI works with single or replica sets
var uri = 'mongodb://localhost/test?poolSize=4';
mongoose.createConnection(uri);

Mongoose's default connection logic is deprecated as of 4.11.0. Please opt in to the new connection logic using the useMongoClient option, but make sure you test your connections first if you're upgrading an existing codebase!

// Using `mongoose.connect`...
var promise = mongoose.connect('mongodb://localhost/myapp', { useMongoClient: true, /* other options */ });
// Or `createConnection`
var promise = mongoose.createConnection('mongodb://localhost/myapp', { useMongoClient: true, /* other options */ });
// Or, if you already have a connection
connection.openUri('mongodb://localhost/myapp', { /* options */ });

This deprecation is because the MongoDB driver has deprecated an API that is critical to mongoose's connection logic to support MongoDB 3.6, see this github issue for more details.

Next Up

Now that we've covered connections, let's take a look at how we can break pieces of our functionality out into reusable and shareable plugins.