I need to merge base record data with two layers of edits in different collections

I have one collection that contains some base record data, for example:

[
  {
    "_id": "one",
    "name": "The Enchanted River",
    "description": "A mystical journey along a forgotten river that holds secrets of a lost civilization.",
    "paperCopy": true,
    "digitalCopy": true,
    "bestSeller": true,
    "isDiscounted": false,
    "discountPercentage": 0.0
  },
  {
    "_id": "two",
    "name": "Galactic Odyssey",
    "description": "An epic space adventure exploring distant galaxies and encountering alien civilizations.",
    "paperCopy": true,
    "digitalCopy": false,
    "bestSeller": true,
    "isDiscounted": false,
    "discountPercentage": 5.57
  },
  {
    "_id": "three",
    "name": "The Last Heir",
    "description": "A gripping tale of a young prince fighting to reclaim his throne against all odds.",
    "paperCopy": true,
    "digitalCopy": true,
    "bestSeller": true,
    "isDiscounted": false,
    "discountPercentage": 27.1
  },
  {
    "_id": "four",
    "name": "Quantum Dreams",
    "description": "A scientist's groundbreaking discovery challenges the very fabric of reality.",
    "paperCopy": true,
    "digitalCopy": true,
    "bestSeller": true,
    "isDiscounted": true,
    "discountPercentage": 22.85
  },
  {
    "_id": "five",
    "name": "The Forgotten Garden",
    "description": "An old gardener unravels the secrets of a mysterious estate with a tragic past.",
    "paperCopy": false,
    "digitalCopy": true,
    "bestSeller": false,
    "isDiscounted": false,
    "discountPercentage": 0.0
  }
]

Admins and users are allowed to make edits where they are stored in a different collection.

[
    {
        "_id": "one",
        "user": "admin",
        "patch": {
           bestSeller: false
        }
    },
    {
        "_id": "one",
        "user": "user1",
        "patch": {
           "bestSeller": true,
           "isDiscounted": true,
           "discountPercentage": 99,
           "frontOfStore": true
        }
    },
]

I need to write an aggregate to merge the base record data with admin edits and then with user edits, add previously undefined fields, with the option to query on them.

1. First $lookup patches by admins, as "adminEdits"

   • if there can be multiple admin edits, add a sort order in the lookup pipeline

2. Then $lookup patches by users, as "userEdits"; I'm treating non-admins as users

   • sort by something if there can be multiple

3. $concatArrays the admin and user patches, and $mergeObjects the resulting array to get one "finalPatch".

   • Decide if user patches should overwrite admin patches, set the order in concatArrays accordingly.
   • This stage will work as-is whether is 1 admin patch + 1 user patch or many admin patches + many user patches (any combination of 0, 1 or many patches).

4. Merge that into the main document and clear out the extra fields.

db.books.aggregate([
  {
    // first get admin patches
    $lookup: {
      from: "edits",
      localField: "_id",
      foreignField: "book_id",
      as: "adminEdits",
      pipeline: [
        { $match: { user: "admin" } }
      ]
    }
  },
  {
    // then get user patches (non-admin)
    $lookup: {
      from: "edits",
      localField: "_id",
      foreignField: "book_id",
      as: "userEdits",
      pipeline: [
        { $match: { user: { $ne: "admin" } } }
      ]
    }
  },
  {
    // concat the admin & user patch arrays
    // decide if user edits overwrite admin (concat order)
    // then merge the array of patches sub-objects
    $set: {
      finalPatch: {
        $mergeObjects: {
          $concatArrays: ["$adminEdits.patch", "$userEdits.patch"]
        }
      }
    }
  },
  {
    // merge the result into the doc
    $replaceWith: { $mergeObjects: ["$$ROOT", "$finalPatch"] }
  },
  {
    // remove all the extra fields created in this pipeline
    $unset: ["adminEdits", "userEdits", "finalPatch"]
  }
])

Mongo Playground

Side note: Your collection with the patches repeats the _id for books which is not permitted for the Primary Key. So I've called it book_id in the example and pipeline.