What is the correct way to associate an existing simple product with an existing configurable product?
I've been struggling with this for a couple of weeks now. I somehow got it to work im 2.0 using the API classes in Magento\ConfigurableProduct\Api, but that's broken in 2.1. The added products don't show up in the configurable's admin page.
I found some blog posts and Stack Overflow questions related to this, but they either don't work or they don't use the API interfaces.