In a project we recently completed, we had to refactor an object so that one of its foreign key associations referred to a new model. We use Ruby/Rails and deploy code to Heroku with Preboot enabled. Implementing this change safely without any downtime was surprisingly tricky to get right (in fact we didn’t at first, and ended up with a brief outage in a part of our product, although fortunately no data was lost). Pedro Belo’s post on hot compatibility lists some patterns to make basic data migrations zero-downtime deployment safe. As Belo mentions at the end of his post, hot compatibility needs to be addressed at the application level, and often takes a lot of planning and work. This article describes a reusable pattern to safely make a change to a foreign key reference with no downtime.