DCI stands for Data, Context and Interaction, and is a programming paradigm that works similarly to OOP, and is easy to implement in languages like Ruby.
An example I came across in this article, explains how we can use mixins to implement DCI in Rails:
user.extend(Buyer)
user.add_to_cart(product_id, quantity)
Here, User contains little or no behavior and is only responsible for data.
A Rails controller can act at the context,
is responsible for injecting the use case or role into the object.