Accepts_nested_attributes_for does not set association cache until it is called using to_a

rubyonrails-talk
1 
ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.string :title
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
    t.string :body
  end
end

class Post < ActiveRecord::Base
  has_many :comments, inverse_of: :post
  
  accepts_nested_attributes_for :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

> post = Post.build(title: 'Intro Post 1', comments_attributes: [{ body: "Comment 1" }, { body: "Comment 2" }] )
> post.comments.count
 => 0
> post.comments.size
 => 2
> post.comments
 => []
> post.comments.class
 => Post::ActiveRecord_Associations_CollectionProxy 
> post.comments.to_a
 => [#<Comment:0x00007fd923e30b30 id: nil, post_id: nil, body: "Comment 1">,
 #<Comment:0x00007fd924df5408 id: nil, post_id: nil, body: "Comment 2">]
> post.comments
 => [#<Comment:0x00007fd923e30b30 id: nil, post_id: nil, body: "Comment 1">,
 #<Comment:0x00007fd924df5408 id: nil, post_id: nil, body: "Comment 2">]