diff --git a/activerecord/lib/active_record/associations/collection_proxy.rb b/activerecord/lib/active_record/associations/collection_proxy.rb
index 776d65294e33c17ad549a10269bd4775f43610d0..bc310cd13ee36317c6dc83aada6bfe89088de4bc 100644
--- a/activerecord/lib/active_record/associations/collection_proxy.rb
+++ b/activerecord/lib/active_record/associations/collection_proxy.rb
@@ -277,7 +277,6 @@ class CollectionProxy < Relation
# person.pets.create!(name: nil)
# # => ActiveRecord::RecordInvalid: Validation failed: Name can't be blank
-
##
# :method: concat
#
@@ -337,7 +336,6 @@ class CollectionProxy < Relation
#
# person.pets.replace(["doo", "ggie", "gaga"])
# # => ActiveRecord::AssociationTypeMismatch: Pet expected, got String
-
##
# :method: delete_all
@@ -531,6 +529,31 @@ class CollectionProxy < Relation
# person.pets # => []
#
# Pet.find(4, 5, 6) # => ActiveRecord::RecordNotFound: Couldn't find all Pets with IDs (4, 5, 6)
+
+ ##
+ # :method: size
+ #
+ # Returns the size of the collection. If the collection hasn't been loaded,
+ # it executes a SELECT COUNT(*) query.
+ #
+ # class Person < ActiveRecord::Base
+ # has_many :pets
+ # end
+ #
+ # # This will execute:
+ # # SELECT COUNT(*) FROM "pets" WHERE "pets"."person_id" = ? [["person_id", 1]]
+ # person.pets.size # => 3
+ #
+ # person.pets
+ # # => [
+ # # #,
+ # # #,
+ # # #
+ # # ]
+ #
+ # # Because the collection is already loaded, this will behave like
+ # collection.size and no SQL count query is executed.
+ # person.pets.size # => 3
##
# :method: empty?