BelongsToReference.meta() sometimes not populated properly

[Herriau]

Description

When an optional belongsTo relationship is loaded independently from the model it's defined on, e.g. when invoking await someRecord.belongsTo('someRel').load(), then the meta hash is only accessible on BelongsToReference object when Adapter.findBelongsTo() resolves to a hash that contains a non-null value on its data key.

Say I have a record loaded through store.findRecord('some-model', <some-id>), intially hydrated from the following JSONAPI document:

GET /some-model/<some-id>

{
  data: {
    "type": "some-model",
    "id": <some-id>,
    "attributes": { ... },
    "relationships": {
      "some-rel": {
        "links": {
          "related": "/some-model/<some-id>/some-rel"
        }
      }
    }
  }
}

Then let's say I load that one relationship with await someRecord.belongsTo('someRel').load(), If the request / response that resulted from that call look like:

GET /some-model/<some-id>/some-rel

{
  "data": {
    "type": "some-other-model",
    "id": <some-other-id>,
    "attributes": { ... },
    ...
  },
  "meta": {
    "foo": "bar",
  }
}

then someRecord.belongsTo('someRel').meta() will return { foo: "bar" }, which is correct. However if the request / response that resulted from that call look like:

GET /some-model/<some-id>/some-rel

{
  "data": null,
  "meta": {
    "foo": "bar",
  }
}

then someRecord.belongsTo('someRel').meta() will return null, when I would have expected it to return { foo: "bar" } as well.

Reproduction

models/car.ts

export default class Car extends Model {
  @belongsTo('navigation-system')
  navigationSystem: NavigationSystem | null;
}

models/navigation-system.ts

export default class NavigationSystem extends Model {}

adapters/car.ts

export default class CarAdapter extends ApplicationAdapter {
  async findAll(store: any, type: any): Promise<any> {
    return {
      data: [
        {
          type: 'cars',
          id: 1,
          attributes: {},
          relationships: {
            navigationSystem: {
              links: {
                related: '/car/1/navigation-system',
              }
            }
          }
        },
        {
          type: 'cars',
          id: 2,
          attributes: {},
          relationships: {
            navigationSystem: {
              links: {
                related: '/car/2/navigation-system',
              }
            }
          }
        }
      ]
    };
  }

  async findBelongsTo(store: any, snapshot: any): Promise<any> {
    switch (snapshot.id) {
      case 1:
        return {
          data: { type: 'navigation-systems', id: 10, attributes: {} },
          meta: { foo: 'bar' },
        };

      case 2:
        return {
          data: null,
          meta: { foo: 'bar' },
        };
    }
  }
}

test.ts

const [car1, car2] = (await store.findAll('car')).toArray();

car1.belongsTo('navigationSystem').meta(); // { foo: 'bar' }
car2.belongsTo('navigationSystem').meta(); // null, expected { foo: 'bar' }

Versions

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[Herriau]

This originally came up when investigating a public way of dealing with this other issue: #5629

[runspired]

A PR with a test for this would be awesome, I suspect the issue is either that our code which ensures that the response of findBelongsTo/findHasMany is a complete payload is insufficient or that due to the missing data/included property it never gets threaded through to the relationship. In either case the fix shouldn't be terribly hard and I'd be happy to pair on a fix.