MongoEngine（燒瓶）不能正確排序（通過聚合）-時不時地

Question

嘗試按兩個字段進行排序，將優先級放在第一個-但在大多數情況下，似乎優先於第二個字段（lastName）。 有時，它可以正常運行-不確定是什么原因做出了更改。

使用Robo3t作為客戶端，並連接相同的數據庫，該命令可以正常工作。

使用以下代碼運行Flask並使用Mongoengine：

pipeline = [{"$sort": {"importance": -1, "lastName": 1}}, {"$match": {"visible": True, "editionId": "2017"}},
            {"$project": {'firstName': 1, 'lastName': 1, 'biography': 1, 'position': 1, 'workAt': 1, '_id': 0,
                          "imageUrl": 1}}]
doc = Speakers.objects.aggregate(*pipeline)

在Robo3t中使用相同的代碼（“ True”變為“ true”除外）：

db.getCollection('speakers').aggregate([{"$sort": {"importance": -1, "lastName": 1}}, {"$match": {"visible": true, "editionId": "2017"}},
            {"$project": {'firstName': 1, 'lastName': 1, 'biography': 1, 'position': 1, 'workAt': 1, '_id': 0,
                          "imageUrl": 1}}], {"collation": {"locale": "en"}}
      )

mongodb（3.4.9）上的索引如下所示：

> db.speakers.getIndexes()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "website_v3.speakers"
    }
]

使用數據庫版本v3.4.9

我什至嘗試創建重要性索引（排序規則），但是完全一樣。 首先它起作用，然后它不起作用。

任何想法歡迎！

編輯：

好像flask_mongoengine弄亂了$ sort和$ project的順序。

mongo
> db.system.profile.find().pretty()
{
    "op" : "query",
    "ns" : "website_v3.system.profile",
    "query" : {
        "find" : "system.profile",
        "filter" : {

        }
    },
    "keysExamined" : 0,
    "docsExamined" : 0,
    "cursorExhausted" : true,
    "numYield" : 0,
    "locks" : {
        "Global" : {
            "acquireCount" : {
                "r" : NumberLong(2)
            }
        },
        "Database" : {
            "acquireCount" : {
                "r" : NumberLong(1)
            }
        },
        "Collection" : {
            "acquireCount" : {
                "r" : NumberLong(1)
            }
        }
    },
    "nreturned" : 0,
    "responseLength" : 98,
    "protocol" : "op_command",
    "millis" : 0,
    "planSummary" : "COLLSCAN",
    "execStats" : {
        "stage" : "COLLSCAN",
        "nReturned" : 0,
        "executionTimeMillisEstimate" : 0,
        "works" : 2,
        "advanced" : 0,
        "needTime" : 1,
        "needYield" : 0,
        "saveState" : 0,
        "restoreState" : 0,
        "isEOF" : 1,
        "invalidates" : 0,
        "direction" : "forward",
        "docsExamined" : 0
    },
    "ts" : ISODate("2017-10-19T20:55:30.415Z"),

}
{
    "op" : "command",
    "ns" : "website_v3.speakers",
    "command" : {
        "aggregate" : "speakers",
        "pipeline" : [
            {
                "$match" : {
                    "visible" : true,
                    "editionId" : "2017"
                }
            },
            {
                "$sort" : {
                    "lastName" : 1,
                    "importance" : -1
                }
            },
            {
                "$project" : {
                    "firstName" : 1,
                    "position" : 1,
                    "biography" : 1,
                    "workAt" : 1,
                    "imageUrl" : 1,
                    "_id" : 0,
                    "lastName" : 1
                }
            }
        ],
        "cursor" : {

        }
    },
    "cursorid" : 168077246844,
    "keysExamined" : 0,
    "docsExamined" : 541,
    "hasSortStage" : true,
    "numYield" : 4,
    "locks" : {
        "Global" : {
            "acquireCount" : {
                "r" : NumberLong(16)
            }
        },
        "Database" : {
            "acquireCount" : {
                "r" : NumberLong(8)
            }
        },
        "Collection" : {
            "acquireCount" : {
                "r" : NumberLong(7)
            }
        }
    },
    "nreturned" : 101,
    "responseLength" : 157635,
    "protocol" : "op_query",
    "millis" : 2,
    "planSummary" : "COLLSCAN",
    "ts" : ISODate("2017-10-19T20:55:57.691Z"),
}
{
    "op" : "getmore",
    "ns" : "website_v3.speakers",
    "query" : {
        "getMore" : NumberLong("168077246844"),
        "collection" : "speakers"
    },
    "originatingCommand" : {
        "aggregate" : "speakers",
        "pipeline" : [
            {
                "$match" : {
                    "visible" : true,
                    "editionId" : "2017"
                }
            },
            {
                "$sort" : {
                    "lastName" : 1,
                    "importance" : -1
                }
            },
            {
                "$project" : {
                    "firstName" : 1,
                    "position" : 1,
                    "biography" : 1,
                    "workAt" : 1,
                    "imageUrl" : 1,
                    "_id" : 0,
                    "lastName" : 1
                }
            }
        ],
        "cursor" : {

        }
    },
    "cursorid" : 168077246844,
    "keysExamined" : 0,
    "docsExamined" : 0,
    "hasSortStage" : true,
    "cursorExhausted" : true,
    "numYield" : 0,
    "locks" : {
        "Global" : {
            "acquireCount" : {
                "r" : NumberLong(4)
            }
        },
        "Database" : {
            "acquireCount" : {
                "r" : NumberLong(2)
            }
        },
        "Collection" : {
            "acquireCount" : {
                "r" : NumberLong(2)
            }
        }
    },
    "nreturned" : 204,
    "responseLength" : 303603,
    "protocol" : "op_query",
    "millis" : 0,
    "planSummary" : "COLLSCAN",
    "ts" : ISODate("2017-10-19T20:55:57.702Z"),
}

Answer 1

MongoDB將按照您指定的確切順序執行聚合階段（大多數情況下）。 $match運算符不對輸出順序提供任何保證。

因此，您可能希望將$match放入管道的開頭（緊隨$sort ），出於性能原因，這是一個好主意。