When to use a “has_many :through” relation in Rails?

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Say you have these models:

Car
Engine
Piston

A car has_one :engine
An engine belongs_to :car
An engine has_many :pistons
Piston belongs_to :engine

A car has_many :pistons, through: :engine
Piston has_one :car, through: :engine

Essentially you are delegating a model relationship to another model, so instead of having to call car.engine.pistons, you can just do car.pistons

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