For a draft enabled model:

```
namespace my;

entity Books {
  key ID : Integer;
  title  : String;
  stock  : Integer;
  author : Association to Authors;
}

entity Authors {
  key ID : Integer;
  name   : String;
  alive  : Boolean;
}

service CatalogService {
    @odata.draft.enabled
    @readonly entity Books as projection on my.Books;

    @readonly entity Authors as projection on my.Authors;
}
```

`my.Books` will have a field:
```
        "IsActiveEntity": {
          "type": "cds.Boolean",
          "key": true,
          "default": {
            "val": true
          },
          "@UI.Hidden": true
        },
```

which is marked as virtual by the lean draft implementation.
This virtual key must not be part of the primary key matching
which is done for a path expression in a `UPDATE.where`:

https://github.com/cap-js/cds-dbs/blob/982b8b796da4b577c5641039d2036816209c0437/db-service/test/cqn4sql/UPDATE.test.js#L73-L97

TODO:

- [] test this

reject all path expressions which navigate along a managed association
which does not provide foreign keys. In this case, we can't construct
join conditions, where exists subqueries or correlated subqueries for expands.