在MongoDB中从数组的数组中投影元素

{
  "_id": 1,
  "array1": [
    [1, 2, 3],
    [4, 5, 6]
  ]
}

db.collection.find({}, {"array1.$": 1})

{
  "_id": 1,
  "array1": [[1, 2, 3]]
}

{
  "_id": 1,
  "array1": [
    [
      [1, 2, 3],
      [4, 5, 6]
    ],
    [
      [7, 8, 9],
      [10, 11, 12]
    ]
  ]
}

db.collection.find({}, {"array1.$.$": 1})

{
  "_id": 1,
  "array1": [[[1, 2, 3], [4, 5, 6]]]
}