Imagine you've just finished setting up a OneToMany relationship in Hibernate, eagerly anticipating a seamless integration of parent and child entities. You run your application, expecting it to work flawlessly, but then you're met with a dreaded exception: 'Column cannot be null'. At first glance, it’s confusing, especially since you may not have directly assigned null values to any columns. Understanding the reasons behind this error and learning how to address it is essential for building robust Hibernate-based applications. In this article, we’ll dive deep into the common causes of this error and explore effective techniques to resolve it with detailed examples and explanations.