http.js:690
    throw new Error('Can\'t set headers after they are sent.');
          ^
Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (http.js:690:11)
    at ServerResponse.header (/home/maschzh/workspace/u9blogsApp/node_modules/express/lib/response.js:666:10)
    at ServerResponse.json (/home/maschzh/workspace/u9blogsApp/node_modules/express/lib/response.js:232:10)
    at Promise.exports.topicOne (/home/maschzh/workspace/u9blogsApp/controllers/topic.js:65:14)
    at Promise.onResolve (/home/maschzh/workspace/u9blogsApp/node_modules/mongoose/node_modules/mpromise/lib/promise.js:177:8)
    at Promise.emit (events.js:95:17)
    at Promise.emit (/home/maschzh/workspace/u9blogsApp/node_modules/mongoose/node_modules/mpromise/lib/promise.js:84:38)
    at Promise.fulfill (/home/maschzh/workspace/u9blogsApp/node_modules/mongoose/node_modules/mpromise/lib/promise.js:97:20)
    at schema (/home/maschzh/workspace/u9blogsApp/node_modules/mongoose/lib/query.js:1400:13)
    at model.Document.init (/home/maschzh/workspace/u9blogsApp/node_modules/mongoose/lib/document.js:254:11)

代码:

exports.topicOne = function (req, res, next){
    Topic.findById(req.params.topicId, function (err, topic){
        if(err){
               return next(err);
        }
        return res.json(topic.doc);
    });
};