MongoDB - Return matched sub-document from an array based on condition

Question

Imagine a document like below -

I want to return the PART along with Input's would be language say "ENG" or "FRA" or "GER"

Document-

{
    "_id" : ObjectId("56a5337f19d312a4156a3625"),
    "PartId" : "Part1",
    "Name" : "Name",
    "active" : "true",
    "attribute1" : "value1",
    "attribute2" : "value2",
    "LanguageData":[ 
        {
            "Language" : "ENG",
            "description" : "PARt1- English",
            "default" : "true"
        }, 
        {
            "Language" : "FRA",
            "description" : "Part1 in french",
            "supported" : "true"
        }, 
        {
            "Language" : "GER",
            "description" : "Part1 In German",
            "supported" : "false"
        }
    ]
}

If input is "ENG"

{
    "_id" : ObjectId("56a5337f19d312a4156a3625"),
    "PartId" : "Part1",
    "Name" : "Name",
    "active" : "true",
    "attribute1" : "value1",
    "attribute2" : "value2",
    "LanguageData":[ 
        {
            "Language" : "ENG",
            "description" : "PARt1- English",
            "default" : "true"
        }
    ]
}

If input is "GER" the out put should not return this document as "supported is false"

{}

If input is say "CHN" then default should be returned -

{
    "_id" : ObjectId("56a5337f19d312a4156a3625"),
    "PartId" : "Part1",
    "Name" : "Name",
    "active" : "true",
    "attribute1" : "value1",
    "attribute2" : "value2",
    "LanguageData":[ 
        {
            "Language" : "ENG",
            "description" : "PARt1- English",
            "default" : "true"
        }
    ]
}

I am not able to return sub document based on the conditions

Only the subdocument from array should be returned based on below conditions

1) So if language is present, then return that sub document.

2) If Sub-Document is not present always return default.

3) If Sub Document is present and not supported , do not return the whole document

Possible duplicate of Retrieve only the queried element in an object array in MongoDB collection — Sede
– Sede, Commented Feb 1, 2016 at 8:22
Can I use condition's in elemMatch during projection, slect specific language if present else select Default if language is not present? — AlokRagate
– AlokRagate, Commented Feb 1, 2016 at 8:30

zangw · Accepted Answer · 2016-02-01 10:54:57Z

2

To query the subdocument, you can do that through aggregate

db.document.aggregate([
    {$match: {'LanguageData.Language': 'GER'}},
    {$project: {
        LanguageData: {$filter: {
            input: '$LanguageData',
            as: 'LanguageData',
            cond: {$eq: ['$$LanguageData.Language', 'GER']}
        }}
    }}
])

For return the default data, the $ifNull could do that

db.document.aggregate([
    {$unwind: '$LanguageData'}, 
    {$match: {'LanguageData.Language': 'GER'}}, 
    {$project: 
       {LanguageData: 
          {Language: {$ifNull: ['$LanguageData.Language', 'CHN']}, 
           description: {$ifNull: ['$LanguageData.description', 'default description']}}}}
]);

edited Feb 1, 2016 at 10:54

answered Feb 1, 2016 at 8:20

zangw

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6 Comments

AlokRagate Over a year ago

Thanks for the reply, but I cannot match default rule when some language is not present "ZHN" is not present in the array of languages.

zangw Over a year ago

@AlokRagate, I think $ifNull may meet your requirement to return the default value.

AlokRagate Over a year ago

Thanks for the response , Although I am getting the below error-uncaught exception: aggregate failed: { "errmsg" : "exception: invalid operator '$filter'", "code" : 15999, "ok" : 0 }

zangw Over a year ago

@AlokRagate, I just give you some sample codes here, for more details of the aggregation parameter, please go to the mongodb site....

zangw Over a year ago

@AlokRagate, the $filter is added after version 3.2, what the version of mongo are you using?

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