$elemMatch with distinct

问题

I have some problems with distinct queries.

db.sessions.distinct("tests.device_serial")
[
        "",
        "5b34f4bf9854a",
        "5b34f4bf98664",
        "5b34f4bf98712",
        "5b34f4bf9876b",
        "5b34f4bf987c6"
]

I don't want to get the result with empty strings. I tried to run query:

 db.sessions.distinct("tests.device_serial", {"tests.device_serial" : {$ne: ""}})
[ ]

Why I got empty array? Where is my mistake?

回答1:

Guessing tests.device_serial is an array, here's your mistake :

 db.sessions.distinct("tests.device_serial", {"tests.device_serial" : {$ne: ""}})

Query in your distinct command is filtering documents where the array 'tests' contains a field named device_serial with a value of "", and not only the fields in array.

To achieve what you want, you can use aggregation framework, unwind array to multiple documents, filter and group by null with an $addToSet command to get distinct values.

Here's the query :

db.sessions.aggregate(
    [
        {
            $unwind: {
                path : "$tests"
            }
        },
        {
            $match: {
            "tests.device_serial":{$ne:""}
            }
        },
        {
            $group: {
              "_id":null,
                "device_serials":{$addToSet:"$tests.device_serial"}
            }
        },
    ]
);

来源：https://stackoverflow.com/questions/51181355/elemmatch-with-distinct