mongoose查询内部文档数组

const User = mongoose.model('User', {
  name: String,
  hobbies: [{ type: String }]
});

User.find({ hobbies: 'reading' }, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

User.find({ hobbies: { $elemMatch: { $in: ['reading', 'swimming'] } } }, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

const Book = mongoose.model('Book', {
  title: String,
  author: { type: mongoose.Schema.Types.ObjectId, ref: 'User' }
});

Book.find({ title: 'The Great Gatsby' })
  .populate('author')
  .exec((err, books) => {
    if (err) {
      console.error(err);
    } else {
      console.log(books);
    }
  });