userId not inserting via args but works with static payload in MongoDB

Hey MongoDB community,

I’m encountering an odd issue. When I manually insert the document into MongoDB with a hardcoded userId (as shown below), it works fine. But when I try to insert using args.userId (coming from my application), the document gets created but without the userId field.

Here’s the actual model data that gets created successfully when I hardcode the userId:

{
    "_id": { "$oid": "650c44c147cb927c814b0c86" },
    "userId": { "$oid": "64ccc1b58bccb41a451a5a60" },
    "sheetId": "BUL5UO7GB9",
    "sheetName": "1695302848827_Yarn.xlsx",
    "categorySlug": "YARN",
    "isDeleted": false,
    "deletedAt": null,
    "createdAt": { "$date": { "$numberLong": "1695302849943" } },
    "updatedAt": { "$date": { "$numberLong": "1695302849943" } },
    "__v": { "$numberInt": "0" }
}

Issue Summary:

• Manual hardcoding:
  When I directly assign userId: "64ccc1b58bccb41a451a5a60", everything works and the document is created with userId.
• Dynamic via args.userId:
  When using userId: args.userId from my backend payload, the userId field does not show up in the document, even though I console.log(args.userId) and it looks fine in the logs.

Things I have tried:

• Validated that args.userId is not undefined or null.
• Confirmed the args.userId is a valid 24-character hex string.
• Converted args.userId to an ObjectId before inserting.
• Logged payload before create() and it looks identical to the hardcoded one.
• Checked for hidden whitespace or encoding issues.

Tech stack:

• MongoDB (using Mongoose ODM)
• Node.js backend

What could be causing this discrepancy?

Would really appreciate any help from the community!