ValidationError on setting null on subdocument field after several other values being set

[ValeriyMaslenikov]

Prerequisites

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

Mongoose version

8.7.1

Node.js version

21.6.2

MongoDB server version

8.0.0

Typescript version (if applicable)

5.4.5

Description

Fully working reproducible example

import mongoose from "mongoose";
const { Schema, model } = mongoose;

async function run() {
  await mongoose.connect("mongodb://localhost:27017/mongoose-test", {});

  mongoose.set("debug", true);

  // Define the nested schema
  const currentMilestoneSchema = new Schema(
    {
      id: { type: String, required: true },
    },
    {
      _id: false,
    }
  );

  const milestoneSchema = new Schema(
    {
      current: {
        type: currentMilestoneSchema,
        required: true,
      },
    },
    {
      _id: false,
    }
  );

  const campaignSchema = new Schema(
    {
      milestones: {
        type: milestoneSchema,
        required: false,
      },
    },
    {
      _id: false,
    }
  );

  const questSchema = new Schema(
    {
      campaign: { type: campaignSchema, required: false },
    },
    {
      _id: false,
    }
  );

  const parentSchema = new Schema({
    quests: [questSchema],
  });

  const ParentModel = model("Parent", parentSchema);

  // Create and save the parent document
  let doc = new ParentModel({
    quests: [
      {
        campaign: {
          milestones: {
            current: {
              id: "milestone1",
            },
          },
        },
      },
    ],
  });

  await doc.save();

  // Set the nested schema to null
  doc.quests[0].campaign.milestones.current = {
    id: "milestone1",
  };
  doc.quests[0].campaign.milestones.current = {
    id: "",
  };

  doc.quests[0].campaign.milestones = null;

  try {
    // Save the document again
    await doc.save();
  } catch (err) {
    // Expected validation error
    console.error("Validation error:", err);
  }

  await mongoose.disconnect();
}

run().catch((err) => console.error(err));

The following example leads into the following output:

Validation error because of `current.id` is required

Mongoose: parents.insertOne({ quests: [ { campaign: { milestones: { current: { id: 'milestone1' } } } } ], _id: ObjectId("6707efc27c06131efbea4e40"), __v: 0}, {})
Validation error: Error: Parent validation failed: quests.0.campaign.milestones.current.id: Path `campaign.milestones.current.id` is required., quests.0.campaign.milestones.current: Path `campaign.milestones.current` is required.
    at ValidationError.inspect (/private/tmp/mongoose-bug-1/node_modules/mongoose/lib/error/validation.js:52:26)
    at formatValue (node:internal/util/inspect:805:19)
    at inspect (node:internal/util/inspect:364:10)
    at formatWithOptionsInternal (node:internal/util/inspect:2298:40)
    at formatWithOptions (node:internal/util/inspect:2160:10)
    at console.value (node:internal/console/constructor:351:14)
    at console.warn (node:internal/console/constructor:384:61)
    at run (/private/tmp/mongoose-bug-1/second.mts:90:13)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  errors: {
    'quests.0.campaign.milestones.current.id': ValidatorError: Path `campaign.milestones.current.id` is required.
        at validate (/private/tmp/mongoose-bug-1/node_modules/mongoose/lib/schemaType.js:1385:13)
        at SchemaType.doValidate (/private/tmp/mongoose-bug-1/node_modules/mongoose/lib/schemaType.js:1369:7)
        at /private/tmp/mongoose-bug-1/node_modules/mongoose/lib/document.js:3071:18
        at process.processTicksAndRejections (node:internal/process/task_queues:77:11) {
      properties: [Object],
      kind: 'required',
      path: 'campaign.milestones.current.id',
      value: null,
      reason: undefined,
      [Symbol(mongoose#validatorError)]: true
    },
    'quests.0.campaign.milestones.current': ValidatorError: Path `campaign.milestones.current` is required.
        at validate (/private/tmp/mongoose-bug-1/node_modules/mongoose/lib/schemaType.js:1385:13)
        at SchemaType.doValidate (/private/tmp/mongoose-bug-1/node_modules/mongoose/lib/schemaType.js:1369:7)
        at SchemaSubdocument.doValidate (/private/tmp/mongoose-bug-1/node_modules/mongoose/lib/schema/subdocument.js:265:35)
        at /private/tmp/mongoose-bug-1/node_modules/mongoose/lib/document.js:3071:18
        at process.processTicksAndRejections (node:internal/process/task_queues:77:11) {
      properties: [Object],
      kind: 'required',
      path: 'campaign.milestones.current',
      value: null,
      reason: undefined,
      [Symbol(mongoose#validatorError)]: true
    }
  },
  _message: 'Parent validation failed'
}

Whereas If I change the required to false on nested subdocs:

  const currentMilestoneSchema = new Schema(
    {
-      id: { type: String, required: true },
+      id: { type: String, required: false },
    },
    {
      _id: false,
    }
  );

  const milestoneSchema = new Schema(
    {
      current: {
        type: currentMilestoneSchema,
-       required: true,
+       required: false,
      },
    },
    {
      _id: false,
    }
  );

then mongoose generates pretty absurd query, which tries to set both subdocuments and their parents to null:

Absurd output

Mongoose: parents.insertOne({ quests: [ { campaign: { milestones: { current: { id: 'milestone1' } } } } ], _id: ObjectId("6707f10577d2fec90b937034"), __v: 0}, {})
Mongoose: parents.updateOne({ _id: ObjectId("6707f10577d2fec90b937034"), __v: 0 }, { '$set': { 'quests.0.campaign.milestones.current.id': null, 'quests.0.campaign.milestones.current': null, 'quests.0.campaign.milestones': null }}, {})
Validation error: MongoServerError: Updating the path 'quests.0.campaign.milestones.current' would create a conflict at 'quests.0.campaign.milestones.current'
    at UpdateOneOperation.execute (/private/tmp/mongoose-bug-1/node_modules/mongodb/src/operations/update.ts:151:32)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at tryOperation (/private/tmp/mongoose-bug-1/node_modules/mongodb/src/operations/execute_operation.ts:263:14)
    at executeOperation (/private/tmp/mongoose-bug-1/node_modules/mongodb/src/operations/execute_operation.ts:109:12)
    at Collection.updateOne (/private/tmp/mongoose-bug-1/node_modules/mongodb/src/collection.ts:362:12) {
  errorResponse: {
    index: 0,
    code: 40,
    errmsg: "Updating the path 'quests.0.campaign.milestones.current' would create a conflict at 'quests.0.campaign.milestones.current'"
  },
  index: 0,
  code: 40,
  [Symbol(errorLabels)]: Set(0) {}
}

But if you go and stop making two sets before setting to null, e.g.

  doc.quests[0].campaign.milestones.current = {
    id: "milestone1",
  };

-  doc.quests[0].campaign.milestones.current = {
-    id: "milestone2",
-  };

  doc.quests[0].campaign.milestones = null;

Then output looks as expected:

Mongoose: parents.insertOne({ quests: [ { campaign: { milestones: { current: { id: 'milestone1' } } } } ], _id: ObjectId("6707f1f01235b106373ed7d2"), __v: 0}, {})
Mongoose: parents.updateOne({ _id: ObjectId("6707f1f01235b106373ed7d2"), __v: 0 }, { '$set': { 'quests.0.campaign.milestones': null } }, {})

Steps to Reproduce

Copy-paste the first code block from description.
Use the following package.json contents:

{
  "name": "mongoose-bug-1",
  "version": "1.0.0",
  "main": "index.js",
  "type": "module",
  "author": "",
  "license": "ISC",
  "description": "",
  "dependencies": {
    "mongoose": "^8.7.1",
    "typescript": "^5.6.3"
  },
  "devDependencies": {
    "@tsconfig/node21": "^21.0.3",
    "tsx": "^4.19.1"
  }
}

Execute it via:

npx tsx ./index.mts

Expected Behavior

I'd expect that I can change the value of field which contains subdocument as many times I need, and after that set null within a single business transaction without validation errors on save.