I think we need to specific another column(s) when sum of column.

[ifbaran]

For example,

We assume we want to sum of "price" with "account_name". I can't get right now. You can take a look given example.

INPUT:

ABCModel::select('account_name')->where('id', '=', $_id)->sum('price');

OUTPUT:

0 => [
  "account_name" => "ACCOUNT A",
  "sum" => 333
],
1 => [
  "account_name" => "ACCOUNT B",
  "sum" => 85
],
2 => [
  "account_name" => "ACCOUNT C",
  "sum" => 51
],

[GromNaN]

The sum method returns the aggregated value.
This issue need an example of input document and expected output in order to write the query.

If the price field contains a list of numbers, then the project method must be used:

ABCModel::select('account_name')
    ->project(['price' => ['$sum' => '$price']])
    ->where('id', '=', $_id)
    ->get();

Documentation of the project method: https://www.mongodb.com/docs/drivers/php/laravel-mongodb/v5.x/query-builder/#show-a-subset-of-fields-and-array-values-in-the-results-example

Documentation of the $sum operator: https://www.mongodb.com/docs/manual/reference/operator/aggregation/sum/#use-in--project-stage