Rails Nested Attributes vs Build from Association

rails-nested-attributes-association.rb

class User < ApplicationRecord
  has_many :posts
  accepts_nested_attributes_for :posts, allow_destroy: true

  has_one :profile
  accepts_nested_attributes_for :profile
end

# To create nested records through the parent:

# Option 1: Nested Attributes
user = User.new(name: 'izumi', posts_attributes: [
                  { title: 'first post' },
                  { title: 'second poast' }
                ])
user.save

# Option 2: Building from the association
user = User.new(name: 'izumi')
user.posts.new([
                 { title: 'first post' },
                 { title: 'second post' }
               ])
user.save

# ----------------------------------------

# update, create and delete nested records at the same time
user = User.find_by(name: 'izumi')
user.update(name: 'izumi 2', posts_attributes: [
              # update first post
              { id: 1, title: 'updated first post' },
              # delete 2nd post
              { id: 2, _destroy: true },
              # create 3rd post
              { title: 'new third post' }
            ])

# ----------------------------------------

# However, with has_one, we can't build from association:
user = User.new(name: 'miyaki')
user.profile.new # <=== error

# We have to use nested attributes:
user = User.new(name: 'miyaki', profile_attributes: { age: 20 })