Mongoose：在嵌套的Schema上无法调用undefined的方法'push'

Question

我设置了我的架构：

var videoSchema = new Schema({
    title: String,
    url: String, //Link to the video
    rating: { type: Number, default: 0},
    description: String //a paragraph or two to go along with the video
});

var tutorialPageSchema = new Schema({
    title: String, //IE Peeking, Clutching, etc.
    tutorials: [videoSchema]
});

var categorySchema = new Schema({
    title: String, //intermediate, beginner, config, map knowledge, etc.
    pages: [tutorialPageSchema]
});

exports.Video = mongoose.model('video', videoSchema);
exports.TutorialPage = mongoose.model('tutorialPage', tutorialPageSchema);
exports.Category = mongoose.model('category', categorySchema);

像这样添加文档：

var newPage = new models.TutorialPage({
    title: req.query.page.title,
    tutorials: []
});
parentCategory.pages.push(newPage);

作为回报，它给了我一个很好的“TypeError：无法调用未定义的方法'push' “

其他时候问的问题是，人们没有按照正确的顺序加载模式，或者在设置模式时将模型与模式混为一谈，但这似乎不是我的问题。是什么给了什么？