Error: Mongoose array discriminator not applied to array elements

Direct answer

Mongoose does not propagate the discriminatorKey to subdocuments stored inside an array field. The reliable pattern is to define the array element base type as its own Schema with a discriminatorKey, register each subtype with .discriminator() on that schema, and push instances created from the discriminator model—not plain objects—into the parent array.

Confirmed behavior vs. likely explanation

Confirmed: In Mongoose 6.x and 7.x, single nested subdocuments honor discriminators, but array elements do not receive the key automatically. When a discriminated subdocument is pushed into an array, the key is omitted and the element is treated as the base type upon retrieval.

Likely explanation: The array casting path does not invoke the same discriminator resolution logic used for single nested paths. As a result, even a base-model instance loses its subtype marker when it is assigned to an array path, because the array caster hydrates elements using the base schema alone.

Steps for this case

1. Extract the array element schema into a standalone Schema and set its discriminatorKey (for example, kind).
2. Register subtypes by calling baseSchema.discriminator('SubtypeName', subSchema) before the parent schema is compiled.
3. In application code, instantiate the specific discriminator model and push that instance into the array. Do not rely on Schema.Types.Mixed, which skips validation and prevents casting back to the subtype.
4. Avoid injecting the key only in pre('save') middleware for large arrays; the casting overhead and risk of bypassing internal logic outweigh the convenience.

Verification

After saving, inspect the raw BSON document to confirm the key exists on every element:

db.parents.findOne({ _id: ObjectId("...") }, { items: 1 })

Then hydrate the document and check the constructor of each array element:

const doc = await Parent.findById(id);
console.log(doc.items[0] instanceof SubtypeModel); // should be true

Assumptions and uncertainty

This guidance assumes Mongoose 6.x or 7.x and that the parent schema references the base element schema in an array path such as [elementSchema]. If a future release adds native array discriminator support, existing documents that lack the key will require a manual migration to backfill the field before the new casting logic can hydrate them correctly. Because this area is subject to ongoing framework changes, validate the exact API surface against your installed Mongoose version before relying on it in production.