Select nested array object and replace it

Question

I got an array (as result of a mongoDB query) with some elements like this:

{ 
    "_id": "ExxTDXJSwvRbLdtpg",
    "content": [
        {
            "content": "First paragraph", 
            "language":"en", 
            "timestamp":1483978498 
        },
        {
            "content": "Erster Abschnitt", 
            "language":"de", 
            "timestamp":1483978498 
        }
    ]
}

But I need to get just a single content field for each data array element, which should be selected by the language. So the result should be (assuming selecting the english content):

{ 
    "_id": "ExxTDXJSwvRbLdtpg",
    "content": "First paragraph"
}

instead of getting all the content data...

I tried to do it with find(c => c.language === 'en), but I don't know how to use this for all elements of the data array. Maybe it is also possible to get the data directly as a mongodb query??

Answer 1

You could iterate the array and replace the value inside.

var array = [{ _id: "ExxTDXJSwvRbLdtpg", content: [{ content: "First paragraph", language: "en", timestamp: 1483978498 }, { content: "Erster Abschnitt", language: "de", timestamp: 1483978498 }] }];

array.forEach(a => a.content = a.content.find(c => c.language === 'en').content);

console.log(array);

Version with check for content

var array = [{ _id: "ExxTDXJSwvRbLdtpg", content: [{ content: "First paragraph", language: "en", timestamp: 1483978498 }, { content: "Erster Abschnitt", language: "de", timestamp: 1483978498 }] }, { _id: "no_content" }, { _id: "no_english_translation", content: [{ content: "Premier lot", language: "fr", timestamp: 1483978498 }, { content: "Erster Abschnitt", language: "de", timestamp: 1483978498 }] }];

array.forEach(function (a) {
    var language;
    if (Array.isArray(a.content)) {
        language = a.content.find(c => c.language === 'en');
        if (language) {
            a.content = language.content;
        } else {
            delete a.content;
        }
    }
});

console.log(array);

Answer 2

Given that _id and language are input variables, then you could use this aggregate command to get the expected result:

db.collection.aggregate([{
  $match: {
    _id: _id,
  }
}, {
  $unwind: '$content'
}, {
  $match: {
    'content.language': language,
  }
}, {
  $project: {
    _id: 1,
    content: '$content.content'
  }
}])