Get first embedded document in an array

Question

I tried to get the first element of an array of documents. As you can see, "primera" fiels is empty in the result:

{
"_id" : ObjectId("5bb164d71d2eef353d292cab"),
"asunto" : {
    "id" : "5b48bb7251b997b3cc23b9e4",
    "abreviatura" : "AF",
    "titulo" : "Alarma Fuente",
    "descripcion" : "Alarma generada por un evento de energía en la fuente."
},
"tiempos" : [
    {
        "cola" : "5b55f8a351b9979f1e417b43",
        "inicio" : 1538352343,
        "fin" : 1538353214,
        "total" : 871,
        "usuario" : {
            "id" : 0,
            "usuario" : "MOICA"
        }
    }
],
"pFecha" : "2018-09-30",
"primera" : [ ]

}

My query is:

var start = ISODate("2018-10-01T00:00:00.000Z");
var end = ISODate("2018-10-01T23:59:59.000Z");

db.moicaTickets.aggregate([

    { $match : { asunto:{$exists:true}, asunto: {$ne:null},tiempos:{$exists:true}, tiempos: {$ne:null},creado:{$exists:true}, "creado.lastEvent_tc": { $gt: start, $lt: end } }},
    { $project: { pFecha: { $dateToString: { format: "%Y-%m-%d", date: "$creado.lastEvent_tc", timezone: "-03:00" } }, asunto: 1, tiempos: 1, primera: "$tiempos.0"}}

         ]).pretty()

Thank you!

What does it mean get the first element of an array of documents. As you can see, "primera" fiels is empty in the result?? Please try to explain it more — Ashh
– Ashh, Commented Dec 12, 2018 at 17:28
"tiempos" is an array of elements (in this case, there is only one document). I want the field "primera" to be the first one of all these documents. — NGabioud
– NGabioud, Commented Dec 12, 2018 at 17:31

Ashh · Accepted Answer · 2018-12-12 17:33:40Z

1

You can use $arrayElemAt to get any indexed element from the array

db.collection.aggregate([
  { "$project": {
    "pFecha": {
      "$dateToString": {
        "format": "%Y-%m-%d",
        "date": "$creado.lastEvent_tc",
        "timezone": "-03:00"
      }
    },
    "asunto": 1,
    "tiempos": 1,
    "primera": { "$arrayElemAt": ["$tiempos",0]}
  }}
])

answered Dec 12, 2018 at 17:33

Ashh

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