The Rails™ 4 Way, Third Edition

Chapter 7. Active Record Associations

Any time you can reify something, you can create something that embodies a concept, it gives you leverage to work with it more powerfully. That’s exactly what’s going on with has_many :through.

—Josh Susser

Active Record associations let you declaratively express relationships between model classes. The power and readability of the Associations API is an important part of what makes working with Rails so special.

This chapter covers the different kinds of Active Record associations available while highlighting use cases and available customizations for each of them. We also take a look at the classes that give us access to relationships themselves.

7.1 The Association Hierarchy

Associations typically appear ...