如何创建自定义对象作为输出，字段名称作为 MongoDB 中的键？

Question

Changing the output of an aggregation query where the key is the field name from the database.

I tried the following: How to use field value as key name in Mongodb result

But it results in the following error:

MongoError: $arrayToObject requires an object keys of 'k' and 'v'. Found incorrect number of keys:1

var data = await Message.aggregate([
    {
      $group: {
        _id: '$message',
        last_message: { $last: '$date_create', },
        conversation: {
          $push: '$$ROOT',
        },
      },
    },
    {
      $project: {
        input: { $arrayElemAt: ['$conversation.message', 0] },
        output: { $arrayElemAt: ['$conversation.mainTopic', 0] },
        _id: 0,
      },
    },
    { $sort: { last_message: -1 } },
  ]);

I want to change the ouput from (current result):

{ "input": "Test", "output": "general" },

TO:

{ "input": "Test", "output": { general: 1, }, },

Answer 1

To convert { "input": "Test", "output": "general" } into { "input": "Test", "output": { general: 1 } } you need $arrayToObject operator which takes either an array of objects with k and v fields or an array of 2-element arrays like below:

db.collection.aggregate([
    {
        $project: {
            _id: 0,
            input: 1,
            output: {
                $arrayToObject: [
                    [
                        [ "$output", 1 ]
                    ]
                ]
            }
        }
    }
])

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