I found this start along the fence line. It's attractive, but I'm concerned that it's something invasive.
Multnomah County Oregon plant identification
Your plant looks like a black locust seedling. Black locusts are native to the eastern US, but are very tough and grow well in adverse conditions, so they do well in Portland. The trees get to be 40-60 feet tall with extensive root systems. This article has additional information with pictures of the mature tree and of the compound leaves, Black Locust http://www.uky.edu/hort/Black-Locust. Unfortunately, they often grow too well and spread through root suckers as well as seeds, so are considered a nuisance tree in Portland, Nuisance Trees in Portland https://www.portlandoregon.gov/trees/article/514066. I would carefully consider whether you want to keep the tree.