Mongo DB中的数组添加

Question

如何在MongoDB中的文档中添加数组？

e.g：

{"_id":1, "a":[1,2,3,4]}

{"_id":2, "a":[3,4,5,6]}

结果：

{"sum":[4,6,8,10]}

Answer 1

请尝试完成aggregation

> db.collection.aggregate([
          // 1. unwind the array a with this array index
          {$unwind: {path: '$a', includeArrayIndex: 'index'}}, 
          // 2. sum them by their index
          {$group: {
                   _id: '$index', 
                   a: {$sum: '$a'}
          }},
          // 3. sort by array index
          {$sort: {_id: 1}},
          // 4. group them into the sum array
          {$group: {_id: null, sum: {$push: '$a'}}}
   ])