node.js - mongoose Schema.statics.func 无法访问到 this

schemas

• book.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var ObjectId = Schema.Types.ObjectId;

var BookSchema = new Schema({
    // some code
});

BookSchema.pre('save',(next) =>{
    console.log(this); // 此处 this 输出为 {}
    // some code
    next();
})
BookSchema.statics = {
    fetch: (cb) => {
        console.log(this); // 此处 this 输出为 {}
        // some code
    }
}

module.exports = BookSchema;

models

• book.js

var mongoose = require('mongoose');
var BookSchema = require('../schemas/book');
var Book = mongoose.model('book',BookSchema);

module.exports = Book;

routes

• books.js

var Book = require('../models/book');
mongoose.connect('mongodb://localhost/readbook');
Book.fetch((err, books) => {
    // some code
});

代码中两处输出均为 {}
请指教, O(∩_∩)O谢谢