MongoDB - 从数组中获取所有元素

Question

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I want to $match multiple items in $facet operator from an dataset and return output in simple array form just like as it is before.

Exception : I don't want to use array-names after $facet-stage anywhere in my aggeration. because it becomes so difficult to handle those in my code. I think I can go with k and v or something.

I hope you understand

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PlayGround Link : https://mongoplayground.net/p/Vw-GkPPHOiw

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Dataset:

[
  {
    "_id": 2,
    "title": "Melancholy III",
    "artist": "Munch",
    "tags": [
      "woodcut",
      "Expressionism"
    ]
  },
  {
    "_id": 4,
    "title": "The Great Wave off Kanagawa",
    "artist": "Hokusai",
    "tags": [
      "woodblock",
    ]
  },
  {
    "_id": 5,
    "title": "The Persistence of Memory",
    "artist": "Dali",
    "tags": [
      "painting",
      "oil"
    ]
  },
  {
    "_id": 7,
    "title": "The Scream",
    "artist": "Munch",
    "tags": [
      "Expressionism",
      "painting",
      "oil"
    ]
  },
]

Query:

db.collection.aggregate([
  {
    $facet: {
      "field1": [
        {
          $match: {
            "artist": "Munch"
          }
        }
      ],
      "field2": [
        {
          $match: {
            "artist": "Grosz"
          }
        }
      ]
    }
  },

//  Below is query which I've tried to make it one array. But I want make it 
//   without using the name "field1" and "field2" anywhere from below after this stage.

//  Is that possible to do??

  {
    $project: {
      items: {
        $concatArrays: [
          "$field1",
          "$field2"
        ]
      }
    }
  }
])

Exprected Output:

   [
      {
        "_id": 2,
        "title": "Melancholy III",
        "artist": "Munch",
        "tags": [
          "woodcut",
          "Expressionism"
        ],
      },
      {
        "_id": 7,
        "title": "The Scream",
        "artist": "Munch",
        "tags": [
          "painting",
          "oil"
        ],
      },
      {
        "_id": 1,
        "title": "The Pillars of Society",
        "artist": "Grosz",
        "tags": [
          "painting",
          "Expressionism",
        ],
      }
   ]

PlayGround : https://mongoplayground.net/p/Vw-GkPPHOiw

Answer 1

You can additionally use unwind and replaceRoot to get the resulted object at root level

[
 {
    $facet: {
      "field1": [
        { $match: { "artist": "Munch" } }
      ],
      "field2": [
        { $match: { "artist": "Grosz" } }
      ]
    }
  },
  {$project: {x: {$objectToArray: '$$ROOT'}}},
  {$unwind: '$x'},
  {$unwind: '$x.v'},
  {$replaceRoot: {newRoot: '$x.v'}}
]