MongoDB Query 对象中 $or 的使用，判断 username 或 email 是否存在。

mongodb_nodejs_signup.js

var userSignup = function (req, res) {
  var signup = req.body;
	// 用户对象
	var muuser = {
		"username": signup.username,
		"password": signup.password,
		"firsttime": (new Date()).getTime(),
		"email": signup.email
	};
	
	var result = {
		"success": false
		"description": ""
	};
	
	db.collection('muusers', function(err, collection) {
		// 判断 username,或 email 是否存在
		var query = {
			$or: [
			{'username': signup.username}, 
			{'email': signup.email}
			]
		};
		
		collection.findOne( , function(err, document) {
			
			assert.equal(null, err);
			if (document == null) {
				// 如果没有记录，则插入一条新的用户记录				
				collection.insert(muuser, {safe: true}, function(err, records){
				    console.log("Record added as " + records[0]._id);
						result.success = true;
				res.send(result);
				res.end();
				});
			} else {
				result.success = false;
				result.description = "username already exists";
				res.send(result);
				res.end();
			}	
		});
	});	
}