create_post_action.rb

## CREATE EXAMPLE ##
# folder: app/actions/posts
class Actions::Posts::CreatePost
  include Actions::Posts::PostAction # In this scenario there is overlap between create & update, so introduce a concern

  def perform!(post, author_scope:)
    validate_action!(post, author_scope: author_scope)
    post.assign_attributes(attributes)
    post.save!
  end
end

# Call
post = thread.posts.new # Note that we have scoped the entry here
action = Actions::Posts::CreatePost.new(author_id: 1, text: 'Revolution Action')
action.perform!(post, author_scope: thread.authors) # Return value is irrelevant as we built the post beforehand. Awesome!
respond_with post

post_action.rb

# folder: app/actions/concerns/posts
module Actions::Posts::PostAction
  extend ActiveSupport::Concern
  include Common::Action # includes ActiveModel::Model + fail (assertion) helpers

  included do
    attribute :author_id, :integer
    attribute :text, :string

    validates :author_id, :text, presence: true
  end

  def validate_action!(post, author_scope:)
    validate!
    fail 'Author is invalid' if author_scope.where(id: author_id).none?
  end
end

update_post_action.rb

## UPDATE ##
# folder: app/actions/posts
class Actions::Posts::UpdatePost
  include Actions::Posts::PostAction

  def perform!(post, author_scope:)
    post.with_lock do
      validate_action!(post, author_scope: author_scope)
      post.update!(attributes)
    end
  end
end

# Call
post = thread.posts.first!
action = Actions::Posts::UpdatePost.new(author_id: 1, text: 'Banana Plixxy Plaxxion')
action.perform!(post, author_scope: thread.authors)
respond_with post