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Accepted
Richard
Asked: 2020-09-08 13:05:55 +0800 CST 2020-09-08 13:05:55 +0800 CST

How to obtain only the fields that are requested, without showing the structure of the document?

  • 772

Document structure example

{
    "_id" : ObjectId("5d4ae50fbdc1cd6351dab49f"),
    "nombre" : "usuario1",
    "Correo" : "[email protected]",
    "playlist" : 
     [ 
        {
            "nombrePlay" : "play1",
            "descPlay" : "descPlay",
            "canciones" : 
            [ 
                {
                    "nombre":"canción1",
                    "año":"07/09/2019"
                },
                {
                    "nombre":"canción2",
                    "año":"07/09/2019"
                }
            ]
        }
    ]
}

What I want

When searching, I am interested in only showing me the data that I am requesting and not the entire structure of the document.

What I want to return is something similar to the following

{
    "nombre":"canción1",
    "año":"07/09/2019"
}

Even something valid would be the following

"canciones" : 
[
    {
        "nombre":"canción1",
    }
]

how i tried

db.users2.find({
    "nombre": "usuario1",
    "playlist.nombrePlay": "play1",
    "playlist.canciones.nombre": "canción1"
},
{
    "playlist.canciones.nombre": "canción1"
})

Result

It shows me only the data that I want, the problem is that it also returns the structure of the document and the other elements of the list . When I only want the element that I am looking for .

{
    "_id" : ObjectId("5d4ae50fbdc1cd6351dab49f"),
    "playlist" : 
     [ 
        {

            "canciones" : 
            [ 
                {
                    "nombre":"canción1",
                    "año":"07/09/2019"
                },
                {
                    "nombre":"canción2",
                    "año":"07/09/2019"
                }
            ]
        }
    ]
}
mongodb

3 Answers

  1. To obtain the desired response you must use aggregations, with the following you would obtain the desired result

    db.tabla.aggregate([
    {
        $match: {"Correo":"[email protected]"} //busqueda
    },
    {
        $project: {rootPlay: { $arrayElemAt: [ "$playlist", 0 ] }} //Se toma el elemento 0
    },
    {
        $replaceRoot: { newRoot:"$rootPlay"} //se reempla la raiz por canciones
    },
    {   
        $project: {canciones:1} // Se deja solo el campo "canciones"
    }    
])

    Result

    { 
    "canciones" : [
        {
            "nombre" : "canción1", 
            "año" : "07/09/2019"
        }, 
        {
            "nombre" : "canción2", 
            "año" : "07/09/2019"
        }
    ]
}

    You could also make the search easier and then in code manipulate the JSON to obtain the values ​​of the "songs" array

    db.tabla.find({"Correo":"[email protected]"},{_id:0, "playlist.canciones": 1});

    Result

    { 
    "playlist" : [
        {
            "canciones" : [
                {
                    "nombre" : "canción1", 
                    "año" : "07/09/2019"
                }, 
                {
                    "nombre" : "canción2", 
                    "año" : "07/09/2019"
                }
            ]
        }
    ]
}
    • 1

  2. To obtain more than one result in the playlist you must use the following

    db.tabla.aggregate([
    {
        $match: {"Correo":"[email protected]", 'playlist.nombrePlay': {$in: ["play1", "play2"]}} //busqueda
    },
    {   
        $project: {"playlist.nombrePlay":1, "playlist.canciones":1, _id:0} // Se deja solo el campo "canciones" y el nombrePlay para identificar la lista
    }    
])

    Result:

    { 
    "playlist" : [
        {
            "nombrePlay" : "play1", 
            "canciones" : [
                {
                    "nombre" : "canción1", 
                    "año" : "07/09/2019"
                }, 
                {
                    "nombre" : "canción2", 
                    "año" : "07/09/2019"
                }
            ]
        },
        {
            "nombrePlay" : "play2", 
            "canciones" : [
                {
                    "nombre" : "canción3", 
                    "año" : "07/09/2019"
                }, 
                {
                    "nombre" : "canción4", 
                    "año" : "07/09/2019"
                }
            ]
        }
    ]
}

    In this case you can't remove the root of the playlist since it is an array.

    • 1
  3. Best Answer

    If I understand what you want correctly, you're going to have to do it with aggregation.

    See if this would work for you:

    user,playlist,song='usuario1','play1','cancion1'

df = prueba.collectionP.aggregate([{'$unwind': '$playlist'},
                                   {'$match': {'nombre': user,
                                               'playlist.nombrePlay': playlist}},
                                   {'$unwind': '$playlist.canciones'},
                                   {'$match': {'playlist.canciones.nombre': song}},
                                   {'$project': {'_id': '$playlist.canciones.nombre',
                                                 'fecha': '$playlist.canciones.año'
                                                 }}
                                   ])

    This will give you a cursor that traversing it will give the following, although it can be changed by modifying the $project stage:

    for d in df:
    print(d)

{'_id': 'canción1', 'fecha': '07/09/2019'}

    On the other hand, in your document example you have put "nombre" : "usuario1"but you have "name": "usuario1"in the code in which you have tried it (I imagine that it will be an error having passed it here).

    Lastly, I have put as output fechainstead of año, since I think it is better to avoid the ñs (as well as the code tildes).

    • 0