Document.prototype.toObject() with discriminators returns all fields and selectively applies getters

[lcrosetto]

Prerequisites

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

Mongoose version

8.9.7

Node.js version

20.18.0

MongoDB server version

8.0.4

Typescript version (if applicable)

No response

Description

After updating a discriminator key in a document with schema discriminators and getters, the Document includes the fields for the current discriminator, as expected. However, Document.toObject() returns all fields, including those defined in a different discriminator schema, but only applies getters to the fields matching the current discriminator.

Steps to Reproduce

The following script will fail when checking the discriminator fields. In particular, the Document contains only the fields relevant to the discriminator, but the object returned by Document.toObject() contains all fields, including those for the other discriminator schema.

      const conn = mongoose.createConnection(..);

      const baseType = 'baseType';
      const type1Key = 'Type1';
      const type2Key = 'Type2';

      const baseSchema = new mongoose.Schema({
        field: String,
        key: String,
      }, {discriminatorKey: 'key', toObject: {getters: true}});
      const dSchema1 = new mongoose.Schema({
        field1: {
          type: String,
          get(value) {
            return `${value} transformed`;
          },
        },
      });
      const dSchema2 = new mongoose.Schema({
        field2: {
          type: String,
          get(value) {
            return `${value} transformed`;
          },
        },
      });
      baseSchema.discriminator(type1Key, dSchema1);
      baseSchema.discriminator(type2Key, dSchema2);

      const TestModel = conn.model(baseType, baseSchema);

      const field = 'field val';
      const field1 = 'field1 val';
      const field1Transformed = `${field1} transformed`;
      const cdm = new TestModel({
        key: type1Key,
        field,
        field1,
      });
      let cd = await cdm.save();
      assert.equal(cd.field2, undefined);
      assert.equal(cd.field1, field1Transformed);
      cd = cd.toObject();
      assert.equal(cd.field, field);
      assert.equal(cd.key, type1Key);
      assert.equal(cd.field1, field1Transformed);
      assert.equal(cd.field2, undefined);

      const field2 = 'field2 val';
      const field2Transformed = `${field2} transformed`;
      await TestModel.updateOne(
        { _id: cd._id },
        {
          key: type2Key,
          field2,
        },
        {overwriteDiscriminatorKey: true},
      );
      let cd2 = await TestModel.findById(cd._id);
      assert.equal(cd2.field2, field2Transformed);
      assert.equal(cd2.field1, undefined);
      cd2 = cd2.toObject();
      assert.equal(cd2.field, field);
      assert.equal(cd2.key, type2Key);
      assert.equal(cd2.field2, field2Transformed);
      assert.equal(cd2.field1, undefined);

      conn.deleteModel(baseType);

Expected Behavior

As shown in the script above, since a Document with a discriminator includes only the fields relevant to the currently set discriminator key (prior to calling toObject()), I would expect the object returned by toObject() to also only include those fields defined in that discriminator schema.

If all fields must be returned by toObject() (even those not relevant to the current discriminator schema) I would expect all getters to have been called.

[vkarpov15]

Unfortunately this is expected behavior, although I admit the fact that cd2.field1 === undefined is an unpleasant gotcha (see #5159, #5111) - you would need to do cd2.get('field1') to see that field1 is not in fact undefined.

The issue is that the following update does not unset field1

      await TestModel.updateOne(
        { _id: cd._id },
        {
          key: type2Key,
          field2,
        },
        {overwriteDiscriminatorKey: true},
      );

You can see the issue more clearly if you add a field that isn't in any schema in this updateOne():

      await TestModel.updateOne(
        { _id: cd._id },
        {
          key: type2Key,
          field2,
          field3: 'taco'
        },
        {overwriteDiscriminatorKey: true, strict: false},
      );

// Later
cd2.field3; // undefined
cd2.get('field3'); // 'taco'
cd2.toObject().field3; // 'taco'

Your best bet would be to explicitly unset field1 in your update. Unfortunately this requires setting strict: false because field1 isn't in the new discriminator type's schema.

      await TestModel.updateOne(
        { _id: cd._id },
        { 
          $set: { key: type2Key, field2 },
          $unset: { field1: 1 }
        },
        {overwriteDiscriminatorKey: true, strict: false},
      );

We will take a closer look at #5159 for our next major release.

[lcrosetto]

If I log cd2 after toObject() is called, I see:

    {
      _id: new ObjectId('67a6659d06947589d6bf4454'),
      field: 'field val',
      key: 'Type2',
      field1: 'field1 val',
      __v: 0,
      field2: 'field2 val transformed',
      id: '67a6659d06947589d6bf4454'
    }

Above, field2 has had its getter called, but field1 has not. This seems unexpected. I believe this is because when the getters are applied, it is filtering the paths based on the schema (which is using the discriminator):

mongoose/lib/document.js

Lines 4176 to 4178 in 16d2407

	function applyGetters(self, json) {
	const schema = self.$__schema;
	const paths = Object.keys(schema.paths);

Could similar logic be applied in toObject()? As a proof of concept, this test passes if I do something similar in $__toObjectShallow().

[vkarpov15]

The fact that field2's setter is called but field1's is not is expected. field1 is not in the schema, so there's no getter to call.

We could add a strict mode analog to toObject() where we can do toObject({ strict: true }) to avoid fields that aren't in the schema, would that help?

[lcrosetto]

The fact that field2's setter is called but field1's is not is expected. field1 is not in the schema, so there's no getter to call.

We could add a strict mode analog to toObject() where we can do toObject({ strict: true }) to avoid fields that aren't in the schema, would that help?

That would be helpful, yes!

[vkarpov15]

I would still recommend running a migration to unset the unexpected fields in MongoDB