Yii: Fixer - how can I create relations between fixtures?

I have two question about Yii2 Fixtures. Does somebody knows how can I create relations between fixtures?

1. How can I create a parent-child fixture relation? The models have parent_id, but I do not know how I can attach the fixture.

2. How can I create belongs-to fixture relation? Like between the category and product models?

I have read the guide but I did not recognize how I can do this. Or does somebody has any good example about this topic?