fix(query): use embedded discriminator schema when filter selects the key with $eq or $in

[breken-ai]

Summary

When the filter picks an embedded discriminator with $eq or a single-value $in instead of a bare value, Mongoose casts with the base subdocument schema. Discriminator-only paths in the update then count as unknown and strict mode removes them. In the positional-update example below, the whole update is dropped: the updateOne() call resolves without an error, but nothing is written.

getEmbeddedDiscriminatorPath() reads the discriminator value from filter['events.kind'] or filter.events.$elemMatch.kind, and only works when that value is the bare string. { $eq: 'Purchased' } is a common way to write the same condition (for example, code that always wraps user input in $eq). With it, getDiscriminatorByValue() finds nothing, so strict mode drops events.$.quantity. Query casting has the same gap: cast() already accepts a single-value $in for embedded discriminators, but $eq leaves discriminator-only paths uncast ('3' stays a string and matches nothing).

This PR adds a small helper, getDiscriminatorValueFromFilter(). It turns 'X', { $eq: 'X' } and { $in: ['X'] } into 'X' and returns anything else unchanged, so a multi-value $in still falls back to the base schema as before. The helper is used:

• in getEmbeddedDiscriminatorPath() for the dotted filter path and the $elemMatch key (update casting),
• in cast() for the embedded discriminator lookup, replacing the inline $in special case,
• in cast() where it picks a discriminator schema from the filter's discriminator key, so $elemMatch: { kind: { $eq: 'X' }, ... } casts with the discriminator schema.

Tests: the new test in test/model.updateOne.test.js fails on master (modifiedCount is undefined because the update is dropped) and passes with this change. The full suite passes locally (4584 passing, 47 pending, mongodb-memory-server), and eslint is clean on the changed files.

Examples

const eventSchema = new Schema({ message: String }, { discriminatorKey: 'kind', _id: false });
const schema = new Schema({ events: [eventSchema] });
schema.path('events').discriminator('Purchased', new Schema({ quantity: Number }, { _id: false }));
const Test = mongoose.model('Test', schema);

const doc = await Test.create({ events: [{ kind: 'Purchased', quantity: 1 }] });

// Works today
await Test.updateOne({ _id: doc._id, 'events.kind': 'Purchased' }, { $inc: { 'events.$.quantity': 2 } });

// Before: the update is cast to {} and nothing is written. After: quantity is incremented
await Test.updateOne({ _id: doc._id, 'events.kind': { $eq: 'Purchased' } }, { $inc: { 'events.$.quantity': 2 } });
await Test.updateOne({ _id: doc._id, events: { $elemMatch: { kind: { $eq: 'Purchased' } } } }, { $inc: { 'events.$.quantity': 2 } });

I found and fixed this with help from an AI coding assistant, then checked the change and the test results myself.