数据库规范化因first_or_create或错误分配而失败

This question is apparently trivial, but I am a newbie in Rails and I can't figure out where I get it wrong. I am populating associated models from an old spreadsheet. Below is a rake import task snippet. Somehow when location (Washington) is the same and already exists in the database for the Smith's post, the new assignment made for a second record This question is apparently trivial, but I am a