How to filter an array at each array element of a MongoDB collection?

Question

MongoDB

There is a collection:

[
  {
    "name": "Alex",
    "cars": [
      {
        "label": "BMW",
        "age": 13,
        "things": [ ... ]
      },
      {
        "label": "Mercedes",
        "age": 8,
        "things": [ ... ]
      }
    ]
  },
  { ... }
]

We need to get a collection without some `thing`s, by condition, using MongoDB aggregations.
Wrote code:

db.matches.aggregate([{
  $project: {
    '_id': '$_id',
    'cars': {
      $map: {
        input: '$cars',
        as: 'car',
        in: {
          'things': {
            $filter: {
              input: '$$car.things',
              as: 'thing',
              cond: {
                $lte: [ '$$thing.cost', 10 ]
              }
            }
          }
        }
      }
    }
  }
}
])

But it requires an explicit enumeration of all stored fields, the list of which in the real collection varies (is unknown). How to be?

Answer 1

Maybe $redact will help you ?