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  1. Core Server
  2. SERVER-29621

$group by Objects (dictionary) is not reliable

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    • Type: Icon: Bug Bug
    • Resolution: Done
    • Priority: Icon: Minor - P4 Minor - P4
    • None
    • Affects Version/s: 3.2.7, 3.4.5
    • Component/s: Aggregation Framework
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    • ALL
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      Create a test collection : 

      /* 1 */
{
    "_id" : ObjectId("594109ada25deacc151b3904"),
    "backends" : [ 
        {
            "port" : 7080,
            "host" : "127.0.0.1"
        }, 
        {
            "host" : "127.0.0.1",
            "port" : 7080
        }, 
        {
            "port" : 7080,
            "host" : "127.0.0.1"
        }
    ]
}

      Execute the aggregate as below :

      db.getCollection('test').aggregate([
{'$unwind': '$backends'},
{'$group': {'_id': '$backends'}}
])

      Results, two documents:

      /* 1 */
{
    "_id" : {
        "host" : "127.0.0.1",
        "port" : 7080
    }
}

/* 2 */
{
    "_id" : {
        "port" : 7080,
        "host" : "127.0.0.1"
    }
}

      Expected results, one document :

      /* 1 */
{
    "_id" : {
        "host" : "127.0.0.1",
        "port" : 7080
    }
}
      Show
      Create a test collection : /* 1 */ { "_id" : ObjectId( "594109ada25deacc151b3904" ), "backends" : [ { "port" : 7080, "host" : "127.0.0.1" }, { "host" : "127.0.0.1" , "port" : 7080 }, { "port" : 7080, "host" : "127.0.0.1" } ] } Execute the aggregate as below : db.getCollection( 'test' ).aggregate([ { '$unwind' : '$backends' }, { '$group' : { '_id' : '$backends' }} ]) Results, two documents: /* 1 */ { "_id" : { "host" : "127.0.0.1" , "port" : 7080 } } /* 2 */ { "_id" : { "port" : 7080, "host" : "127.0.0.1" } } Expected results, one document : /* 1 */ { "_id" : { "host" : "127.0.0.1" , "port" : 7080 } }
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      Hi,
      Until now, I was doing aggregate and was implementing sort-of DISTINCT by using $group on several documents. It works fine until one of the document got rewritten with its field in a different order.
      Consequence of it, with $foo as an object, you can't assume 

      $group: {'_id': '$foo'}

      is reliable.
      If it's the normal behavior, could we mention this in the docs?
      Thanks in advance

            Assignee:
            mark.agarunov Mark Agarunov (Inactive)
            Reporter:
            JonathanHuot Jonathan Huot [X]
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            Watchers:
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              Created:
              Updated:
              Resolved: