Assume you have a Lookup or Master-Detail relationship defined in the metadata and now you need a simple UI.
In Visualforce lookup-controls on the UI run out-of-the-box using
<apex:inputField ... />
The output is an edit-field together with a looking-glass icon to open a lookup-popup. It's also possible to manually input the beginning of the name of the object which should been looked up, save it, and the platform assigns it.
How to do the same in Lightning Components?
I'm not expecting any source-code here. I would be interested if there is a way to do it natively or if there is a smarter workaround or if not: is this on the roadmap?