When upgrading a Rails application, you might find that sometimes functionality is extracted from Rails and moved into a new gem. These gems are called shims, and they will basically allow you to keep using an old functionality, once the core API takes that out. You can also find shims in form of monkey patches. In this case it’s functionality that you develop to make your migration easier.
In this article I will list some of the functionality of past versions of Rails that was extracted into gems.
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