Ask HN: Has someone migrated an app from MongoDB to Postgres without downtime?

I have an application using MongoDB and I want to migrate this application to use a relational database. Its a Rails application using Mongoid as ORM. Currently, I'm seeing these kinds of problems: - Migrate all the data to the new database and schema - Prepare the application to use the new database - Turn the application to write and read from the new database using the new schema

So, anyone has done it without downtime?