unicloud多表联查

        const db = uniCloud.database()
		let res = await db.collection('order')
		  .aggregate()
		  .lookup({ 
		    from: 'book', //要联查哪个表
		    localField: 'book_id', //本表字段
		    foreignField: '_id',  //关联字段
		    as: 'book_info'  //别名
		  })
		  .match({ //关联表查询条件
		    book_info: {
		      title: '三国演义'
		    }
		  })
		  .project({ //关联别名
		    'book_info.title': true,
		    'book_info.author': true,
		    quantity: true
		  })
		  .end()