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  1. Core Server
  2. SERVER-90398

Improve error message when using $count with _id key

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    • Type: Icon: Improvement Improvement
    • Resolution: Unresolved
    • Priority: Icon: Trivial - P5 Trivial - P5
    • None
    • Affects Version/s: None
    • Component/s: None
    • Labels:
    • Query Optimization

      Running $count using _id as key returns the following error:

      test> db.foo.aggregate([{$count: "_id"}])
MongoServerError[Location15948]: a group's _id may only be specified once

      This is because {$count: X} is just sugar for

      { $group: { _id: null, X: { $sum: 1 } } },
{ $project: {_id: 0}}

      Error is confusing and could be improved

            Assignee:
            sopho.kevlishvili@mongodb.com Sopho Kevlishvili
            Reporter:
            renato.riccio@mongodb.com Renato Riccio
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              Created:
              Updated: