Get only the document if a property exist in other document using Aggregate

Question

So for example i have these items in my collection named borrow:

[
   {
      _id: ObjectId("628ebcc10944a1223397b057"),
      borrower_Id: "6278d1b6b4b7659470572e19",
      borrowedbook_Id: "62710ac63ad1bfc6d1703162",
      borrowStatus: "pending",
      borrowDate: ISODate("2022-05-25T23:33:21.849Z"),
      
    },
    {
      _id: ObjectId("628d9c0b9a3dc72f4aa72f1a"),
      borrower_Id: "6278d1b6b4b7659470572e19",
      borrowedbook_Id: "62710ac63ad1bfc6d170314d",
      borrowStatus: "pending",
      borrowDate: ISODate("2022-05-25T03:01:31.416Z"),
      
    }
 ]

and in this document i want to only get the items where the borrowedbook_Id is existing in other collection named books, How can i achieve this with aggregate method, I want to make sure that I only get the borrow records on books that is existing

books collection:

[
    {
        "_id": "62710ac63ad1bfc6d1703162",
        "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"
    },
]

if I try to query borrow.find({}) , the result should only be

 {
        "_id": "62710ac63ad1bfc6d1703162",
        "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"
    }

since the other borrow record has its borrowedbook_Id missing on other collection

Answer 1

It is not clear of you want to get the borrow record (written) or the book record (expected result example), but in both cases you can use a $lookup:

db.borrow.aggregate([
  {
    $lookup: {
      from: "books",
      let: {book_id: "$borrowedbook_Id"},
      pipeline: [
        {$match: {$expr: {$eq: ["$_id", "$$book_id"]}}},
        {$project: {_id: 1}}
      ],
      as: "existingBook"
    }
  },
  {
    $match: {$expr: {$gte: [{ $size: "$existingBook"}, 1]}}
  },
  {
    $unset: "existingBook"
  }
])

See how it works on the playground example