ActiveRecord::Associations::AssociationProxy

This is the root class of all association proxies:

  AssociationProxy
    BelongsToAssociation
      HasOneAssociation
    BelongsToPolymorphicAssociation
    AssociationCollection
      HasAndBelongsToManyAssociation
      HasManyAssociation
        HasManyThroughAssociation
           HasOneThroughAssociation

Association proxies in Active Record are middlemen between the object that holds the association, known as the @owner, and the actual associated object, known as the @target. The kind of association any proxy is about is available in @reflection. That‘s an instance of the class ActiveRecord::Reflection::AssociationReflection.

For example, given

  class Blog < ActiveRecord::Base
    has_many :posts
  end

  blog = Blog.find(:first)

the association proxy in blog.posts has the object in blog as @owner, the collection of its posts as @target, and the @reflection object represents a :has_many macro.

This class has most of the basic instance methods removed, and delegates unknown methods to @target via method_missing. As a corner case, it even removes the class method and that‘s why you get

  blog.posts.class # => Array

though the object behind blog.posts is not an Array, but an ActiveRecord::Associations::HasManyAssociation.

The @target object is not \loaded until needed. For example,

  blog.posts.count

is computed directly through SQL and does not trigger by itself the instantiation of the actual post records.