[Enh]: Support relationships between views (and tables).

What

Allow entities backed by views to participate in relationships.

Supported combinations

• view → view
• view → table
• table → view

No new configuration syntax. Existing entities.<name>.relationships settings continue to work.

Unlimited combinations

flowchart LR
    Table1
    Table2
    View1
    View2

    Table1 --> Table2
    Table1 --> View1
    View1 --> View2
    View1 --> Table2

[!NOTE]
Stored procedures cannot be related.

Why

Today relationships only work between table entities.

However, DAB already allows relationships to be defined in configuration without requiring database FK constraints. If a relationship can be described using fields in configuration, the engine should be able to generate the join even when the underlying object is a view.

This enables common read models where views shape the data exposed through the API. A view such as SeriesActors can already be exposed as an entity, but it cannot currently participate in relationships.

Example

View → table relationship

Exactly the same syntax as before

"entities": {
  "BooksView": {
    "source": { "object": "dbo.vw_books_details", "type": "view" },
    "relationships": {
      "publisher": {
        "cardinality": "one",
        "target.entity": "Publisher",
        "relationship.fields": [ "publisher_id:id" ]
      }
    }
  }
}

View → view relationship

Exactly the same syntax as table

"relationships": {
  "authors": {
    "cardinality": "many",
    "target.entity": "AuthorsView",
    "relationship.fields": [ "id:book_id" ]
  }
}