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

No response

Reproduction

Not needed

https://loopback.io/doc/en/lb4/Polymorphic-relation.html の Polymorphic relation ドキュメントから始め、そこにある Delivery の例を issue で報告されたフィールドと比較します。関係が deliverableId と deliverableType を使用するように例を更新し、ドキュメント化された例が存在しない deliverable フィールドを要求しなくなったことを確認します。