Make CollectionProxy#proxy_association public API

Co-authored-by: Petrik <petrik@deheus.net>

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -928,7 +928,20 @@ def include?(record)
         !!@association.include?(record)
       end
 
-      def proxy_association # :nodoc:
+      # Returns the association object for the collection.
+      #
+      #   class Person < ActiveRecord::Base
+      #     has_many :pets
+      #   end
+      #
+      #   person.pets.proxy_association
+      #   # => #<ActiveRecord::Associations::HasManyAssociation owner="#<Person:0x00>">
+      #
+      # Returns the same object as <tt>person.association(:pets)</tt>,
+      # allowing you to make calls like <tt>person.pets.proxy_association.owner</tt>.
+      #
+      # See Associations::ClassMethods@Association+extensions for more.
+      def proxy_association
         @association
       end
 

guides/source/association_basics.md

@@ -2827,6 +2827,12 @@ class Supplier < ApplicationRecord
 end
 ```
 
+Extensions can refer to the internals of the association proxy using these three attributes of the `proxy_association` accessor:
+
+* `proxy_association.owner` returns the object that the association is a part of.
+* `proxy_association.reflection` returns the reflection object that describes the association.
+* `proxy_association.target` returns the associated object for `belongs_to` or `has_one`, or the collection of associated objects for `has_many` or `has_and_belongs_to_many`.
+
 ### Association Scoping using the Association Owner
 
 The owner of the association can be passed as a single argument to the scope