'Adding Core Data entity to existing model with relationship to existing entity

I have an existing app that uses Core Data. It has an entity with various attributes, ie Book has an authorName (and other attributes).

I want to create a new entity called Author that has an attribute authorName which has a To Many relationship to Book and Book has a To One relationship to Author (pretend that books have only one author for the sake of discussion).

In an existing project with data, how can I populate the Author entities from the Book?

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'Adding Core Data entity to existing model with relationship to existing entity

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