Migrating a legacy database into an existing Rails app

Sometimes, you need to connect to a legacy database and retrieve some data. Perhaps you are deep in the process of rewriting an application, or maybe your company has bought another company, leaving you, the poor lone developer, responsible for transferring all the data into the existing app. In general, a process like this is considered a a data pipeline and involves extracting data from a foreign source, potentially transforming it, and then loading it into your destination of choice. This process is usually referred to as ETL. Whatever your reason for migrating legacy data, there are many commercial tools out there claiming to help with the task. These tools often come at a cost and tend to focus on more than just one-off migration tasks. They also address data sources such as CSV files, S3 buckets and APIs. In the Rails community, there is an excellent gem called kiba, which has both an open-source and a commercial licence, and focuses on such recurring processes.