Black holes are where God divided by zero.
In Chapter 3 you learned how to manage a changing model with versioning, mappings and migration techniques. This chapter will exercise your newfound migration knowledge as it introduces further changes to the model. The topics will cover expanding beyond one or two entities with the introduction of relationships and, at the end of the chapter, entity inheritance. The implications of delete rules will be discussed, along the impact some of them can have on data validation. The next section will show how to handle data validation errors, or worst case crash gracefully and give users an error code to report back to you.
Relationships link entities. ...