StrictModeError: Throw when setting discriminator field

[abarriel]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.6.3

Node.js version

16

MongoDB server version

4.9.1

Description

Hello, I have a throw error on kind, this is a working exemple, event when I set overwriteDiscriminatorKey to true

StrictModeError: Field kind is not in schema and strict mode is set to throw.

I think I found the issue castUpdate!L47 where overwriteDiscriminatorKey is check only after strict throw.

it only happens when you pass the updateObj directly as a second params, otherwise when we pass with $set: updateData it go through castUpdate.js!L47

if (schema.options.strict === 'throw' && obj.hasOwnProperty(schema.options.discriminatorKey)) {
    throw new StrictModeError(schema.options.discriminatorKey);
  } else if (!options.overwriteDiscriminatorKey) {
    delete obj[schema.options.discriminatorKey];
  }

Steps to Reproduce

const mongoose = require("mongoose");

mongoose.set("strict", "throw");

const options = { discriminatorKey: "kind" };
const EventSchema = new mongoose.Schema({ name: String }, options);
const Event = mongoose.model("Event", EventSchema);

async function main() {
  await mongoose.connect("mongodb://localhost:27017");

  const eventDoc = await Event.findByIdAndUpdate(
    { _id: mongoose.Types.ObjectId() },
    {
        name: "name",
        kind: "kind",
    },
    {
      overwriteDiscriminatorKey: true,
    },
  );

  console.log(eventDoc);
}

main();

Expected Behavior

No response

[abarriel]

I came up with this kind of patch, where I allow developers to fetch and udpate full document and skip the strictCheck when the value of discriminator key is the same as the current model

--- a/lib/helpers/query/castUpdate.js
+++ b/lib/helpers/query/castUpdate.js
@@ -43,7 +43,7 @@ module.exports = function castUpdate(schema, obj, options, context, filter) {
     return obj;
   }
 
-  if (schema.options.strict === 'throw' && obj.hasOwnProperty(schema.options.discriminatorKey)) {
+  if (schema.options.strict === 'throw' && obj.hasOwnProperty(schema.options.discriminatorKey) && obj[schema.options.discriminatorKey] !== schema.discriminatorMapping.value) {
     throw new StrictModeError(schema.options.discriminatorKey);
   } else if (!options.overwriteDiscriminatorKey) {
     delete obj[schema.options.discriminatorKey];
@@ -196,7 +196,7 @@ function walkUpdatePath(schema, obj, op, options, context, filter, pref) {
       }
     }
 
-    if (schema.discriminatorMapping != null && key === schema.options.discriminatorKey && !options.overwriteDiscriminatorKey) {
+    if (schema.discriminatorMapping != null && key === schema.options.discriminatorKey && !options.overwriteDiscriminatorKey && schema.discriminatorMapping.value !== obj[key]) {
       if (strictMode === 'throw') {
         const err = new Error('Can\'t modify discriminator key "' + key + '" on discriminator model');
         aggregatedError = _appendError(err, context, key, aggregatedError);