Rails 5.1 added :default option to belongs_to

From the [changelog](https://github.com/rails/rails/blob/5-1-stable/activerecord/CHANGELOG.md)

![image](https://user-images.githubusercontent.com/3908/30759940-d56de402-9f9d-11e7-9070-54f2db54109d.png)

Now if only someone would add it to `has_one` relationships. Tim says it's harder because then you're setting attributes on another object.

obie-fernandez
September 22, 2017