The field "$name" must be an accumulator object

Question

I have a query and when I use a $group a error shows "the field "$name must be an accumulator object", if if remove the filed "$name" all works well and i have tried to use only "name" instead of "$name" and the error continues.

   User.aggregate([
    {
      $match: {
        "storeKey": req.body.store        
      }
  },
  {
      $group: {
          "_id": "$_id",          
          "name": "$name",              
          "count": {
              "$sum": 1
          },
          "totalValue": {
              "$sum": "$value"
          }      
      }
  },
  {
    $sort: sort
  },
  {
     $skip: req.body.limit * req.body.page
  },
  {
     $limit: req.body.limit
  }
])...

Solution

There are some aggregation operators that can only be used in $group aggregation and named as $group accumulators

Just as you used $sum here you have to use for the name key as well

{ "$group": {
  "_id": "$_id",
  "name": { "$first": "$name" },  //$first accumulator
  "count": { "$sum": 1 },  //$sum accumulator
  "totalValue": { "$sum": "$value" }  //$sum accumulator
}}

Accumulator is like array of Elements its Accumulates as Array. $first -> gives 1st name that goes in the group of names

Example: so if you have $_id same but different name ["Darik","John"] specifying $first will give Darik & similarly $last will give John