Polymorphic Relation - Wrong Doc

Describe the bug

Hi there!

Let's suppose a base Delivery model with 3 fields: id, deliverableType, and deliverableId.
If you follow the docs, you'll get this error when requesting some deliverable relations:

Request GET /deliveries?filter=whatever failed with status code 500. Error: ER_BAD_FIELD_ERROR: Unknown column 'deliverable' in 'field list'

This is caused by this part of code, as you're asking it to search on the corresponding datasource for the field "deliverable" which doesn't exist:

class Delivery extends Entity {
@hasOne(() => Deliverable, {polymorphic: true})
  deliverable: Deliverable;

deliverableType: string;
}

Instead, it should be like this (search deliverableId on deliverableType's model):

class Delivery extends Entity {
@hasOne(() => Deliverable, {polymorphic: true})
  deliverableId: string;

deliverableType: string;
id: string;
}

With that change, polymorphic relation is working as expected (assuming repositories are correctly configured)

Logs

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Additional information

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Reproduction

Not needed

Bắt đầu với tài liệu Polymorphic relation tại https://loopback.io/doc/en/lb4/Polymorphic-relation.html và so sánh ví dụ Delivery trong đó với các trường được issue báo cáo. Cập nhật ví dụ để quan hệ sử dụng deliverableId và deliverableType, sau đó xác minh rằng ví dụ được ghi trong tài liệu không còn yêu cầu trường deliverable không tồn tại.