Converting string to Array (Aggregate mongodb)

Question

Im getting random data from my database using the aggregate but apparently some of them have their genre values in string not in array, How can i convert those random data genre from string to array?

const book = await Book.aggregate([
       /* This is just for testing which books has genres in specific type
       { 
        $match: { genre: { $type: "string" } } 
       },
       */
 
      {   
        $sample: { size: 6 } 
      }
]);

sample data result(genres that are in type of string)

[
    {
        "_id": "62710ac63ad1bfc6d17030fe",
        "title": "Birth of a Theorem",
        "author": "Villani, Cedric",
        "genre": "mathematics",
        "publisher": "Bodley Head",
        "dateOfPublication": "2002-02-28T00:00:00.000Z",
        "noOfCopies": 16,
        "type": "Book",
        "form": "Non-fiction",
        "isbn": "979-81202-479229-867673-6",
        "dateAdded": "2002-11-28T00:00:00.000Z"
    },
    {
        "_id": "62710ac63ad1bfc6d1703108",
        "title": "All the President's Men",
        "author": "Woodward, Bob",
        "genre": "history",
        "publisher": "Random House",
        "dateOfPublication": "2018-02-19T00:00:00.000Z",
        "noOfCopies": 56,
        "type": "Book",
        "form": "Non-fiction",
        "isbn": "978-41428-6606587-937631-",
        "dateAdded": "2011-02-23T00:00:00.000Z"
    },
]

sample data result(genres that are in type of array)

[
    {
        "_id": "62710ac63ad1bfc6d17030be",
        "title": "Superfreakonomics",
        "author": "Dubner, Stephen",
        "genre": [
            "economics",
            "computer_science"
        ],
        "publisher": "HarperCollins",
        "dateOfPublication": "2003-10-06T00:00:00.000Z",
        "noOfCopies": 31,
        "type": "Thesis",
        "form": "Non-fiction",
        "isbn": "978-35029-7186192-465859-7",
        "dateAdded": "2009-02-12T00:00:00.000Z"
    }
]

Answer 1

$sample is for documents instead of embedded array.

Update

db.collection.update({
  genre: {
    $type: "string"
  }
},
[
  {
    $set: { genre: [ "$genre" ] }
  }
],
{
  multi: true
})

mongoplayground

---

Aggregate

db.collection.aggregate([
  {
    $match: {
      genre: {
        $type: "string"
      }
    }
  },
  {
    $set: {
      genre: {
        $cond: {
          if: { $eq: [ { $type: "$genre" }, "string" ] },
          then: [ "$genre" ],
          else: "$genre"
        }
      }
    }
  }
])

mongoplayground