1 db.collection('数据库1').aggregate()
 2   .match({
 3     _id: 'XXX'
 4   })
 5   .lookup({
 6     from: '数据库2',
 7     localField: '数据库1的数组',
 8     foreignField: '_id',
 9     as: 'bookList',
10   })
11   .end()