在Node.js中发送标头后，无法设置标头

Question

我试图了解以下代码的工作原理以及后面的代码是什么。

var Todo = require('./models/database');

function getTodos(res){

    Todo.find()
        .sort('-created_at')
        .exec(function(err, todos) {
            if (err) return console.error(err);

            // ISSUE HERE
            res.json(todos, function(err){
                if (err) console.log(err);
            });     // return all todos in JSON format
        });
};

module.exports = function(app) {

    // api ---------------------------------------------------------------------
    // get all todos
    app.get('/api/todos', function(req, res) {
        getTodos(res);
    });


    // update a todo
    app.get('/api/todos/:todo_id/:replacedText', function(req, res) {
        console.dir(req.params.todo_id);
        console.dir(req.params.replacedText);

        Todo.update({ _id : req.params.todo_id}, 
                {$set: {text: req.params.replacedText}}, 
                function(err, todo) {
                    if (err) res.send(err);

                    getTodos(res);
                });
    });


    // application -------------------------------------------------------------
    app.get('*', function(req, res) {
        res.sendfile('./public/index.html'); // load the single view file (angular will handle the page changes on the front-end)
    });
};

以上代码有效。但当我把Todo.find（）下的代码作为：

// ISSUE HERE
res.json(todos); // return all todos in JSON format

这没有用。我收到以下错误：

http    http.js:689
    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:689:11)
    at ServerResponse.header (\node_modules\express\lib\response.js:595:10)
    at ServerResponse.send (\node_modules\express\lib\response.js:143:12)
    at ServerResponse.json (\node_modules\express\lib\response.js:229:15)
    at Promise.<anonymous> (\app\routes.js:70:12)
    at Promise.<anonymous> (\node_modules\mongoose\node_modules\mpromise\lib\promise.js:177:8)
    at Promise.emit (events.js:95:17)
    at Promise.emit (\node_modules\mongoose\node_modules\mpromise\lib\promise.js:84:38)
    at Promise.fulfill (\node_modules\mongoose\node_modules\mpromise\lib\promise.js:97:20)
    at Promise.resolve (\node_modules\mongoose\lib\promise.js:114:23)

帮助我弄清楚，这里发生了什么。