Where does ActiveRecord::Associations::CollectionProxy get the .each instance method?

Question

Let's say I have models Topics and Posts, where a Topic has_many :posts and a Post belongs_to :topic. I already have some stuff in my database at this point.

If I go into the rails console and type

Topic.find(1).posts

I believe I get back a CollectionProxy object.

=> #<ActiveRecord::Associations::CollectionProxy [#<Post id:30, ......>]>

I can call .each on this to get an Enumerator object.

=> #<Enumerator: [#<Post id: 30, ......>]:each>

I'm confused as to how CollectionProxy is handling .each. I realize that it's inherited at some point but I've been reading the API docs and they don't make it very clear what CollectionProxy is inheriting from unless I'm missing something obvious.

This page doesn't seem to tell me much, and neither does this page.

Answer 1

ActiveRecord::Associations::CollectionProxy is inherited from Relation, and Relation forwards each and many other methods to to_a.

From activerecord/lib/active_record/relation/delegation.rb#L45

delegate :to_xml, :to_yaml, :length, :collect, :map, :each, :all?, :include?, :to_ary, :join, to: :to_a

See Understanding Ruby and Rails: Delegate for an excellent explanation on how delegate works.

Answer 2

Why don't you try asking it where it comes from?

> ActiveRecord::Associations::CollectionProxy.instance_method(:each).owner
=> ActiveRecord::Delegation 

The UnboundMethod#owner method:

Returns the class or module that defines the method.

so each comes from ActiveRecord::Delegation. And if you look at ActiveRecord::Delegation, you'll see this:

delegate ..., :each, ... , to: :to_a

so each is further punted to to_a.each.