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   } ])... 

Answer 1

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