Swift Array Mapping Question

I have an array that contains objects, and those objects have arrays that I want to collapse into a single array.

Basically, I want to be able to remove 2 for-in loops and replace them with a line of code (if I can while maintaining readability).

How would you write this? See https://gist.github.com/jsorge/140d42e710b5f39f59ad for the code.

Update: Not 2 minutes after a @brentsimmons retweet, Andy Matuschak gave me the answer: