A Guide to Active Record Associations

This guide covers the association features of Active Record. By referring to this guide, you will be able to:

• Declare associations between Active Record models
• Understand the various types of Active Record associations
• Use the methods added to your models by creating associations

Chapters

1. Why Associations?
2. The Types of Associations
   • The belongs_to Association
   • The has_one Association
   • The has_many Association
   • The has_many :through Association
   • The has_one :through Association
   • The has_and_belongs_to_many Association
   • Choosing Between belongs_to and has_one
   • Choosing Between has_many :through and has_and_belongs_to_many
   • Polymorphic Associations
   • Self Joins
3. Tips, Tricks, and Warnings
   • Controlling Caching
   • Avoiding Name Collisions
   • Updating the Schema
   • Controlling Association Scope
   • Bi-directional Associations
4. Detailed Association Reference
   • belongs_to Association Reference
   • has_one Association Reference
   • has_many Association Reference
   • has_and_belongs_to_many Association Reference
   • Association Callbacks
   • Association Extensions

1 Why Associations?

Why do we need associations between models? Because they make common operations simpler and easier in your code. For example, consider a simple Rails application that includes a model for customers and a model for orders. Each customer can have many orders. Without associations, the model declarations would look like this: