Retrieve all fields when using $project

I’m using the following specification in $project but I’ve to use field: 1 to retrieve the fields that I want in the final document but instead what I want is to do is include all the fields in the final document.

{
  "newField": {
    "$filter": {
      "input": "$example",
      "as": "example",
      cond: {
        "$and": [
            {"$eq": ["$$example.id", "1234"]}
          ]
      }
    }
  },
}

Output:

{
  _id:ObjectId("5f4d78952ebfba2559305679")
  example: [{_id:: 12314}]
}

Is there any way to retrieve all the fields?

Thank you so much.

If you using $project to simply add a field while you want all other fields you should take a look at https://docs.mongodb.com/manual/reference/operator/aggregation/addFields/ introduced in 3.4.

There is one extra thing that need to be tested. It is to use $project but to exclude a non-existent, which in principle will include all existing fields.