mongo DB 查询，如何 select 一个特定的元素

Question

I have a question for you,

I have some document in my collection "q-1", I would like to extract one element of the table "data".

to do this, I would like to retrieve the one with the id_propositions field at 19

Do you have any idea?

thx in advance

Gautier

Answer 1

One way to accomplish this is to use $unwind , which flattens arrays.

Consider the following ( you can also check out a live demo here ):

Database

[
  {
    _id: 1,
    data: [
      {
        id_propositions: 10,
        desc: "a"
      },
      {
        id_propositions: 19,
        desc: "b"
      },
      {
        id_propositions: 19,
        desc: "c"
      },
      {
        id_propositions: 10,
        desc: "d"
      }
    ]
  },
  {
    _id: 2,
    data: [
      {
        id_propositions: 20,
        desc: "z"
      },
      {
        id_propositions: 19,
        desc: "x"
      },
      {
        id_propositions: 19,
        desc: "y"
      },
      {
        id_propositions: 20,
        desc: "w"
      }
    ]
  }
]

Query

db.collection.aggregate([
  {
    $unwind: "$data"
  },
  {
    $match: {
      "data.id_propositions": 19
    }
  }
])

Result

[
  {
    "_id": 1,
    "data": {
      "desc": "b",
      "id_propositions": 19
    }
  },
  {
    "_id": 1,
    "data": {
      "desc": "c",
      "id_propositions": 19
    }
  },
  {
    "_id": 2,
    "data": {
      "desc": "x",
      "id_propositions": 19
    }
  },
  {
    "_id": 2,
    "data": {
      "desc": "y",
      "id_propositions": 19
    }
  }
]